Proton can be used to run non-Steam games. The standard way to do it is to add the game to Steam via "Games | Add a non-Steam Game to my Library" and then enable Proton after it is added.

But Proton can also be run outside of Steam.

You can add any other executable into Steam and run it on proton that way. I used it for non-game applications with great success. Proton can also be installed and used without Steam, although the process was so involved last time I checked it was easier to use Steam as a launcher even if you have no need for it otherwise.

Yeah building proton yourself is a tad involved. Very doable, but typically a lot easier to just "add" the executable to steam as a non-steam game.

Oh, that's good to hear.