SteamOS isn't publicly available and is very unlikely to be fully open source if/when that changes. Your best bet is probably either Bazzite or Cachy (I would say use Bazzite if you still feel like a noob).

Or just get shit running in Mint; it's very doable. But when it comes to non-steam games, Bazzite is better at working "out of the box," as it were.