Hello Xbox people, I'm a Nintendo gamer with a slice of PlayStation, but have opted to take the plunge for the current gen by buying a series S. Please may I have some recommendations to get my Xbox journey started. So far I have picked up AC Vallhalla, Gears 5, Sunset overdrive and Necromunder hired gun. I'm open to suggestions especially games that have been updated for

series X/S

Just get gamepass, great selection of games.

Since you said you're a Nintendo gamer get Rare Replay.
The Xbox Series S has access to various Xbox and Xbox 360 titles. So if you haven't played these titles I'll recommend Metal Gear Rising, Sonic Adventure 1+2, Sonic Unleashed, Sonic Generations, Panzer Dragoon Orta, Castlevania Symphony of the Night, and some more. I also hope they add more titles.
As for Xbox One titles there's a lot, but I start by saying Resident Evil 2, Bayonetta (also can play Xbox 360 version), Vanquish (also can play Xbox 360 version), Monster Hunter World, Dragon Quest 11 S, Octopath Traveler, Nier Automatta, DOOM, DOOM Eternal, and more.

Congrats first of all. What kind of genre's do you enjoy playing? The easy thing to suggest is getting Game Pass. They currently have $1 for 3 months of Game Pass Ultimate. If you prefer buying games, check out MCC, Forza Horizon 4, Gears Tactic if you enjoy strategy games, Ori 1 and 2, Quantum Break, and a bunch of 3rd party titles have received FPS boost. You can also play plenty of great BC X360 and Xbox titles for cheap.

Here is a list of fps boosted titles you can check out. Games Featuring FPS Boost - Xbox's Major Nelson

I play a variety of genres as I like to broaden my horizons. I'm covered for JRPGs on Switch and it's my go to for indies. I got my series s to catch up on exclusive games I missed and to see multiplats running at 60fps, so thank you for the fps boost list. 

Start picking up the Xbox One heavy hitters...

* Halo 5.
* Halo: The Master Chief Collection.
* Gears 4.
* Gears 5.
* Gears Ultimate Edition.
* Sunset Overdrive.
* Halo Wars Definitive Edition
* Halo Wars 2
* Ori and the will of the wisps.
* Ori and the Blind forest.
* Quantum Break.
* Rare Replay.
* Killer Instinct.
* Gears Tactics.

I would even go back to the Xbox 360 and grab a few titles from there like Fable Anniversary and Fable 2.

Play the Ori games if you like platformers at all, play Halo: CE (via MCC) and Halo 5 whether you like FPSs or not. You just need to experience that franchise since you're a gamer. Gears 5 is a great one too. Also play some Forza (both Motorsport and Horizon).

Why not Series X? Though plenty of reasons come to mind.

I have the Ori games on Switch and they are incredible. Gears 5 I started a few hours ago and love what I've played so far, the boost to series S looks beautiful.  I'm thinking maybe Forza next. 

Affordability is the main reason tbh. Being a Switch and PS4 owner it's been hard to convince my wife to let me buy another device to tether to our TV ?. Seriously though the series S is great value, especially with gamepass.