First-person-shooters with great single player content on the Xbox

You have an Xbox, therefore I'm going to immediately recommend Xbox Game Pass. It's a great deal to get access to a ton of games. Most of my recommendations are going to pull from Game Pass, but it's no coincidence that these games are on there. They're good and Game Pass is worth it even if you just buy a month to check them out.

Alien: Isolation - This is first-person horror, but it's the best Alien game. There is plenty of combat in it, but also a lot of first-person hiding.

Battlefield series - These are all mainly multiplayer, but they also have fairly decent single player campaigns too. 3/4 are modern warfare, 1 is WW1, and 5 is WW2.

Destiny 2 - This game really blurs the single/multiplayer line. They have single player campaigns that can be done in coop too. There are open areas, where you'll see other players and can join in with them in group fights. It also has some dedicated coop and PVP content. It's a great variety of locations and stuff to do and a constantly updating/changing game.

Doom 2016/Doom Eternal - Fast action, shooting demons, run and gun. Eternal gets a bit overly complex with systems, but Doom is awesome.

Halo: Master Chief Collection - If you haven't played Halo, you should check it out. The MCC has six sizeable, fun single player campaigns. Even if you've played Halo, checking it out again in 4K/60fps is fun.

Shadow Warrior/Shadow Warrior 2 - These are a reboot of the old Build engine Shadow Warrior, minus a lot of racist bullshit. Very solid action games, with the second one being a bit more fantasy than the first.

Wolfenstein: The New Order/The New Colossus/Youngblood - This series starts in WW2 and then jumps up to the 60's/70's/80's respectively. Great FPS action, though Youngblood is coop focused. It's the weakest of the three, but I still had a lot of fun with it solo.

Halo has its campaigns, and I'm sure Call of Duty has a single player mode somewhere.

Edit: Stretches the definition of shooters, but if you never played Portal or Portal 2, you're in for a fun time.

What about RPG shooters?

Fallout 4 or (idk how well it runs on S, check that first) Cyberpunk?

You can also play Fallout 3 or NV, they're better RPGs but worse shooters.

Deus Ex.

Both dishonored games are great (although only kinda a shooter)

The bioshock series

I'm not the best person to ask about modern shooters but I will chime in regarding:

I don't care for multiplayer in shooters. I'm usually surrounded by highly skilled players that seemingly do nothing besides dedicating their lives to becoming an expert in that game.

I'm not sure of an xbox equivalent but there is an attempted solution to this problem. Games like Red Orchestra or Squad minimize individual player impact with larger teams, slower gameplay and less gamified movement abilities to try and reward things like positioning over hand eye coordination.

Have you played the Borderlands series yet? I still fire it up the second on PC regularly and I played through Borderlands 1 twice on my 360. The best part is that there are so many side quests, you can really squeeze a lot of play-time out. Combine that with the different character classes and DLC and you have a ton of value in my opinion.