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You can add non-Steam installers to your Steam library, and then select a version of Proton to run them under.

Works for StarCraft II.

You can add non steam games to steam and use proton that way

I would try the program 'bottles' via flatpak. Really easy to use and has run all my non-steam games flawlessly.

Yes, AFAIK there is Bottles and Lutris, but I’m struggling with staring games through them. Perhaps it’s just me though.