Mint is by far one of the most user friendly GUI based Linux distros out there. It's pretty frequently the most recommended intro Linux based OS out there for a reason.

For strictly Debian-based, I think popOS and Mint are probably the best two choices. Ubuntu is fine but I don't care for snaps personally and so those two distros taking care of the legwork of pulling out the Snap-related bits is nice. Pop having an ISO with Nvidia proprietary drivers pre-installed is nice too.

That said, I don't know that I agree with the direction system76 is taking Cosmic, the DE they're planning to replace GNOME with, and so once Cosmic is shipping I think I'll probably prefer Mint between the two thanks to Cinnamon's more traditional desktop leanings.

Personally I prefer Fedora to either in most circumstances, primarily due to having much newer but still reasonably stable and not cutting edge packages, which is nice for getting the improvements in developing tech like Wayland more quickly as well as better support for newer hardware. That's ruled out by the Debian requirement, though.

I’ve been running Pop!_os for a couple of years now as my daily driver. Ultimately though, my home machine is mostly just a game platform and web browser. I’ve really been happy with it, though!

Critical question: does the PC have an nvidia GPU?

I definitely recommend Pop!OS for your purposes, as it makes a lot of gaming-related things go smoother than on vanilla ubuntu. Though I am biased in that I think mint's default DE, Cinnamon, is really ugly, so that influenced my choice. I also have an nvidia graphics card, so that made Pop!OS a no-brainer for me -- if you have an Nvidia GPU, ABSOLUTELY use Pop!OS.

You will have to assess what you think of Cosmic once it's properly released, but it's only in alpha now, so you'll have plenty of time until then -- and even if you dislike it, Pop!OS 22.04 will be supported until 2027 so you have time to keep using it while you decide on something else.

When I swapped the motherboard on my computer, I lost my Windows license and Microsoft support was useless.

Other people have already given good answers regarding Linux (which I definitely think you should try out), but I also wanted to address the Windows activation issue:

https://github.com/massgravel/Microsoft-Activation-Scripts/

I've used this to (permanently) activate various editions of Windows. It works well, instructions are easy to follow, and is regularly maintained. Using it, you won't need to deal with the watermark when you dual boot Windows.

I would prefer to not have to use the terminal often

Thankfully, the terminal can be made less-painful to use for when you have to use it. For advice on how to do that, I'd recommend reading a previous comment I wrote:

if [[ -r "${XDG_CACHE_HOME:-$HOME/.cache}/p10k-instant-prompt-${(%):-%n}.zsh" ]]; then
  source "${XDG_CACHE_HOME:-$HOME/.cache}/p10k-instant-prompt-${(%):-%n}.zsh"
fi
export ZSH="$HOME/.oh-my-zsh"
export FZF_DEFAULT_COMMAND="find . -not -path '*/\.git/*' -printf '%P\\n'"
export FZF_DEFAULT_OPTS="-m --height 100% --layout=reverse --border --color=16 --preview='bat --style=numbers --color=always {}'" # This line requires bat: https://github.com/sharkdp/bat
export ZSHZ_DATA="$HOME/.config/zshz/z"
zstyle ':fzf-tab:*' fzf-flags -m --height 100% --layout=reverse --border --color=16 --preview='bat --style=numbers --color=always {}'
ZSH_THEME="powerlevel10k/powerlevel10k"
POWERLEVEL9K_PROMPT_ON_NEWLINE=false
POWERLEVEL9K_LEFT_PROMPT_ELEMENTS=(host dir status dir_writable vcs)
POWERLEVEL9K_RIGHT_PROMPT_ELEMENTS=()
POWERLEVEL9K_HOST_LOCAL_FOREGROUND="000"
POWERLEVEL9K_HOST_LOCAL_BACKGROUND="255"
POWERLEVEL9K_DIR_HOME_BACKGROUND="005"
POWERLEVEL9K_DIR_HOME_FOREGROUND="000"
POWERLEVEL9K_DIR_HOME_SUBFOLDER_BACKGROUND="004"
POWERLEVEL9K_DIR_HOME_SUBFOLDER_FOREGROUND="000"
POWERLEVEL9K_DIR_DEFAULT_BACKGROUND="001"
POWERLEVEL9K_DIR_DEFAULT_FOREGROUND="000"
POWERLEVEL9K_DIR_ETC_BACKGROUND="003"
POWERLEVEL9K_DIR_ETC_FOREGROUND="000"
POWERLEVEL9K_DIR_PATH_HIGHLIGHT_FOREGROUND="000"
POWERLEVEL9K_DIR_PATH_HIGHLIGHT_BOLD=true
POWERLEVEL9K_DIR_PATH_SEPARATOR_FOREGROUND="000"
POWERLEVEL9K_VCS_CLEAN_FOREGROUND="000"
POWERLEVEL9K_VCS_CLEAN_BACKGROUND="002"
POWERLEVEL9K_VCS_UNTRACKED_FOREGROUND="000"
POWERLEVEL9K_VCS_UNTRACKED_BACKGROUND="003"
POWERLEVEL9K_VCS_MODIFIED_FOREGROUND="000"
POWERLEVEL9K_VCS_MODIFIED_BACKGROUND="009"
DISABLE_AUTO_UPDATE="true"
COMPLETION_WAITING_DOTS="true"
HIST_STAMPS="yyyy/mm/dd"
fpath=($HOME/.config/zsh/completion $fpath)
autoload -Uz compinit && compinit -i
plugins=(
  copybuffer
  fzf-tab
  colored-man-pages
  git
  history-substring-search
  web-search
  zsh-autopair
  zsh-autosuggestions
  zsh-completions
  zsh-syntax-highlighting
  zsh-z
)
source $ZSH/oh-my-zsh.sh
source $HOME/.oh-my-zsh/custom/plugins/fzf-autopair/autopair.zsh
autopair-init
autoload zcalc
source /usr/share/fzf/key-bindings.zsh # This might be Arch Linux specific?

$HOME/.oh-my-zsh
$HOME/.oh-my-zsh/custom/plugins/fzf-autopair
$HOME/.oh-my-zsh/custom/plugins/fzf-tab
$HOME/.oh-my-zsh/custom/plugins/zsh-autosuggestions
$HOME/.oh-my-zsh/custom/plugins/zsh-completions
$HOME/.oh-my-zsh/custom/plugins/zsh-history-substring-search
$HOME/.oh-my-zsh/custom/plugins/zsh-syntax-highlighting
$HOME/.oh-my-zsh/custom/plugins/zsh-z
$HOME/.oh-my-zsh/custom/themes/powerlevel10k

[.oh-my-zsh/custom/plugins/fzf-autopair]
checkout = git clone 'https://github.com/hlissner/zsh-autopair.git' 'fzf-autopair'

[.oh-my-zsh/custom/plugins/fzf-tab]
checkout = git clone 'https://github.com/Aloxaf/fzf-tab.git' 'fzf-tab'

[.oh-my-zsh/custom/plugins/zsh-autosuggestions]
checkout = git clone 'https://github.com/zsh-users/zsh-autosuggestions.git' 'zsh-autosuggestions'

[.oh-my-zsh/custom/plugins/zsh-completions]
checkout = git clone 'https://github.com/zsh-users/zsh-completions.git' 'zsh-completions'

[.oh-my-zsh/custom/plugins/zsh-history-substring-search]
checkout = git clone 'https://github.com/zsh-users/zsh-history-substring-search.git' 'zsh-history-substring-search'

[.oh-my-zsh/custom/plugins/zsh-syntax-highlighting]
checkout = git clone 'https://github.com/zsh-users/zsh-syntax-highlighting.git' 'zsh-syntax-highlighting'

[.oh-my-zsh/custom/plugins/zsh-z]
checkout = git clone 'https://github.com/agkozak/zsh-z.git' 'zsh-z'

[.oh-my-zsh/custom/themes/powerlevel10k]
checkout = git clone 'https://github.com/romkatv/powerlevel10k.git' 'powerlevel10k'

[.oh-my-zsh]
checkout = git clone 'https://github.com/ohmyzsh/ohmyzsh.git' '.oh-my-zsh'

Please ask if you have any questions

I'll go the other side, you should pick up Fedora, it's fast, incredibly stable, and a great starting point to other systems. Fedora tends to be closer in terms of "how linux works", with the polish of modernity. From there, you can decide to keep going with it, try a different version, or just into other OS' like Arch or Nix after you get comfortable. Fedora is IMO one of most user friendly and sensical choices these days. Not only that, but I found it a lot better in terms of gaming than anything Debian based. The comfort you have with Debian will lead you to prefer the comfort of Fedora, its solid, and most things work similar to Debian, with a lot of more stability.

Now for desktop environment, I suggest KDE because it's solid, has a lot of features, and won't be such a major change as GNOME (GNOME is inherently anti-choice, and it's a mess if you want simple things, even their plugin environment is one of their weakest points and it's practically a reqm to use an OS, as an OS, and not a dumbed down phone). Whereas GNOME tried to be Apple (not just the company, but more that old worm in a car artwork), KDE is like a sleek futuristic vehicle, that out of the box works, looks great, and has too many options. OOB most similar to Windows, but just....Better.

I also have an old desktop running proxmox, with various VMs, primarily a NAS running openmediavault. Also, I took a college class on Linux system admin, which focused on various tasks on ubuntu. So overall, I am pretty familiar with Debian-based Linux

So this is like, a non-trivial amount of Linux experience. Because of that, and despite you saying you would prefer a Debian-based distribution, I'd tentatively recommend investigating Arch Linux. I think you're qualified to decide for yourself if it would be "right" for you, so I'll leave out the pros and cons list.

If you like doing server stuff, tinkering with things, and infinite learning curves, there's also NixOS. I'd recommend trying it in a VM or on a spare computer first though. NixOS is generally the most stable, reliable, and resilient Linux distribution, but it's also the most complex. It's a ton of fun to build and orchestrate home servers with, and, once you've got a lot of experience with it, can be a really good option for personal/desktop use.

Similar UI to Windows 10 is preferable

You probably want something that ships with KDE instead of GNOME then. Many popular distros have official KDE variants. I would recommend Kubuntu for your use case.

Manjaro is another option, their KDE flavor is configured to mimic modern Windows very closely. However, it is a bleeding edge rolling release distro that requires some occasional trips to the terminal or some config files to fix things when they inevitably break. A good OS for tinkerers and hobbyists but not one I recommend as a daily driver for casual users.

So I'm not quite sure I recommend this, but I'll share my setup and let you decide.

I use Debian. It is the debian-based distro, after all.

I cannot recommend this if you want a streamlined, beautiful, cohesive experience out-of-the box. I actually prefer that, since it means each component is relatively unchanged, correctly aligns with online documentation, and is easy to swap in/out with other components I prefer

Steam, PlayOnLinux, and Wine all work fine.

Installing arbitrary .deb packages and adding apt repositories works fine.

If you enable non-free firmware, Nvidia works as fine as on any other distro. Although, admitedly, I haven't tested Debian/Nvidia in some years. I'm sure Pop!OS is easier out-of-the-box.

Cinnamon, KDE, XFCE, MATE, etc. all work fine. I currently use i3. It's all fine and easy to switch without reinstalling. I have not tested Gnome 3 as I dislike it. You can probably get some DE to look like Win 10, but I don't have concrete advice here.

I currently use Mint Cinnamon (Cinnamon is the flagship desktop environment choice for Linux Mint) as my daily driver on my primary desktop, and would absolutely recommend that as a very transition friendly first Linux distro. That is not to paint it as 'for beginners / not advanced', that would give the wrong impression. I've been on Linux only for something like a decade now and have stuck with Mint as my preferred distro.

The other possible recommendation I might give is MX Linux - it has gotten quite popular in recent months, Mint and MX Linux are constantly trading places for the #1 popularity spot on distrowatch.

I've recently installed MX Linux on my lab bench computer to try it out, and while I don't have nearly as many hours of use-time with MX, so far I like it.

Both in my experience are very 'it just works' initial experience in terms of absolute minimum troubleshooting to get it up and running, if any at all.

So it’s not Debian based, but I’ve been using CachyOS for the last few months and I really like it. It’s an Arch based distribution that comes with a nice installer and by default uses the KDE Plasma desktop. I think Plasma is one of the best options for anybody migrating from Windows. You should feel right at home.

Libre Office is what I plan to switch to from MS Office

Heads up, I recently discovered Inkscape after suffering far too long using LibreOffice Draw. Inkscape is a direct spiritual successor to old CorelDraw. It's been proving invaluable for SVG and layout work.

Otherwise, Libreoffice is pretty good these days.

Avoid "Long Term Stable" distros. They get updates very slowly so if you find a bug, which is probably already fixed, you have to wait for year or two to get it fixed. They're good for server use or enterprise use, but I would never have one on my home system.

I recommend Fedora KDE because it actually gets updates. Kubuntu non-LTS might also work really well.

Debian's own KDE live image is pretty nice for a complete desktop experience:
https://cdimage.debian.org/debian-cd/current-live/amd64/iso-hybrid/debian-live-12.7.0-amd64-kde.iso

Many people suggested against Debian in the past because its separation of non-free caused some headaches but now that's been included in the release builds it is no longer an issue.

PopOS is fine too if you don't mind Gnome's interface, I'll be more excited for it when they switch to Cosmic though.