best option for a bare-bones message board/forum?

hello tildes,

i am searching for a bare-bones forum/message board. i'll describe what i need and perhaps someone can point me in the right direction:

must have the ability to

• create an account
• create topics with the ability to reply
• fully customizable front-end (html/css) so i can make it match my website
• either self-host or use a custom domain on ext hosting

unimportant but nice to have

• display images in posts via external urls
• different boards (categories?) to post into (would be nice but not vital)
• nested replies (or some visual way to see what you're replying to)

free would be ideal (dreams are nice) but probably wouldn't want to spend more than $5/month as this is a supplement to a website where i don't expect much traffic at all due to the nature of the project. no images/video/audio will be hosted ideally.

i'm ok with "hacker news" or "reddit-like" software but my backend knowledge if very limited. for instance, i looked into self-hosting lemmy but i don't understand how to setup docker. options like phpbb would be fine if i could strip away everything aside from the above-mentioned features.

i want it to be extremely simplistic in appearance and for the end-user (tildes is a great example of the simplistic end-user experience). tildes could be a cool option but i think getting it setup is a bit out of my skill-range.

can anyone offer any suggestions/guidence?

edited to emphasize important aspects and clarify things that are not vital but would be nice to have.