I asked this a while ago which is how I discovered Beyond All Reason which has been my FOSS game of choice as of late.
I’d also recommend Naev and Endless Sky (Both are based on the Escape Velocity Series, Naev is getting a 3D PBR renderer in the next release). Mindustry is good fun, actually purchased this one on steam to support the amazing developer. Extreme Tux Racer is a bit of fun and Super Tux Kart seems to get better with every update (did I mention it can run on the Nintendo Switch via homebrew!)
Edit: I forgot about 0ad and Minetest which I used to play a bit of a while back