Gamepad support relies on the game but it’s v configurable on the deck. You’ll be able to play OpenTTD fine but games like 0AD are always gonna be better on mouse and keyboard.
Minetest is similar - You can configure really good fps controller configs using the steam input system.
You CAN use non-steam distros (it’s just a fancy single board computer after all) but you’ll have a worse time with support. Though The steam deck control drivers are in the linux kernel (and Valve has been very good with upstreaming as they use a bunch of linux contractors to develop SteamOS) so maybe it wouldn’t be so bad. I’d recommend against it unless you were gonna use the deck for tinkering and stuff primarily.