For Red Dead 2, take your time and play it slow. Just enjoy the world, rather than powering through it. Xbox One supports backwards compatibility too, so if you can put up with previous-generation graphics, pick up the 360 copy of Red Dead Redemption and you'll get the full story as well. If you're playing these then avoid spoilers for both.
Forza series is split into two different series of games. Forza Motorsport deals with the more realistic circuit racing. It isn't a simulation, but it "believable" racing, is a good way of putting it. Real cars on (mostly) real circuits, and traditional lap racing. The most recent is Forza Motorsport 7. Don't worry about DLC for it - there's a lot. But it's perfectly fine without it.
Forza Horizon series is less realistic. It's open world exploration in a driving format. Often point to point racing, in a less realistic way as you blast over fields and through small towns. It's fun and easier than Forza Motorsport 7. Latest game is Forza Horizon 4, and you can sign up to the Xbox Game Pass, and the game is part of that.
Hitman is an interesting game. There's been lots of them over the years, but it was recently rebooted into a different type of game. Don't worry about the story (there basically isn't one), but you get 6 levels. Each level has someone you must assassinate in it. You can't just run in and shoot them- there are guards, witnesses, etc. You'll have to work out how. You can change disguises to get into restricted areas, murder people in "accidents" such as drowning them, or having them get hit by an object. There are hundreds of ways of doing it, and each level is detailed enough to present you with many options. It's a slow, patient game, but very rewarding. Once you complete the assissination for a level, you get hundreds of side missions for that level too - assissinating othes, which dramatically changes how you'll approach the level.
Hitman is part of the Xbox Game Pass as well, so if you sign up you can try it for free.
https://www.xbox.com/en-GB/xbox-game-pass
I'd heavily recommend at least trying this pass, because some of the games in it are excellent. Games to look at:
- Wolfenstein
- Forza Horizon 4
- The Division
- Tomb Raider
- Hitman
- Doom
I'd recommend playing all of those games personally. Doom is especially old school fun. Wolfenstein is great fun too. The Division got a hard time by the community, but I loved it.