You mean games that arent available in steam at all, vor those which you haven’t bought there? I found the easiest workaround for me was to simply add the games to my steam library and to launch it from there. Then I don’t have to worry about what proton version I should use or whatnot.

I can also recommend Libretube or Clipious for Android

They need something to become sustainable. They’ve been making losses for years. But thata definetly not the way. Other ways would be to lock some future features behind paywalls, and / or to increase their cut of the sales price per unit sold in the future.