^linked video is of a nice mountain run in Rigs Of Rods
I know everybody always talks about BeamNG, but Rigs Of Rods is fully open source, free and still in development! It isn't rapid development, but I feel like this game is mature enough as a sandbox that this shouldn't be a priority for players deciding whether to try out Rigs Of Rods or not. This game has been around for a LONG time and there is a lot of custom vehicles for it that had a lottt of care and love put into them that makes this game worth checking out beyond any considerations for how quickly features are progressing. I really hope that Rigs Of Rods becomes more broadly appreciated in part because why let all of those cool contributions go to waste?
The graphics aren't amazing but you know what... that makes Rigs Of Rods a great game for a Steam Deck or similar Linux handheld!
https://www.rigsofrods.org/
For similar Free and Open Source Linux racing fun check out the great Stunt Rally, it has splitscreen!
https://stuntrally.tuxfamily.org/