I don't get it but for some reason the 3d open world multiplayer shooter with vehicles and long view distances that runs the best on the steam deck (other than a very graphically simple game like battlebit) is DayZ. I obviously haven't tried every game out there though...

On the V++ deathmatch server (using steamdeck screen not higher resolution monitor) I can be in the middle of a firefight with 20+ people nearby and get a stable 50fps. For DayZ that is basically the most challenging it gets for game performance, in the normal game having 20+ people nearby has to be an extremely rare situation I imagine. Yeah you have to turn the graphics down but you really don't have to sacrifice view distance and the game looks fine on lower graphics. (Gun scopes are very important for playing on a low resolution screen like this though, dont bother playing on servers where gun scopes are extremely rare).

Unfortunately the game does crash randomly every once in awhile, I can't figure out why yet but I am playing on heavily modded servers and honestly the performance is SO MUCH better than any other open world multiplayer shooter like this that I don't really feel like I can even complain much.

Interestingly the engine DayZ was made off of, Arma 3, runs much worse. I guess you can get Arma 3 to play ok offline (it looks like crap after you turn everything down though) but in multiplayer forget it.

Give it a try! Yeah yeah... mouse and keyboard players are gods of aim blah blah blah, you can compete fine with joysticks + gyro if you take the time to get acclimated to it. Go play a deathmatch server an mess around until you feel comfortable.