If issue #1 is about MangoHUD, you should read its manual to find out how to set the overlay size. If it’s not MangoHUD… you should switch to MangoHUD, it’s a fantastic application.

Issue #2: Gamescope has built-in support for FSR1 and NIS upscaling. In the command line parameters, use this: gamescope -W 1920 -H 1080 -w 3840 -h 2160 -F fsr – %command%.

Alternatively, you can use Lossless Scaling for either upscaling or frame generation. It works on Linux and is well-liked, but I’ve never personally used it.

Issue #3:

The default Proton version is the latest one available. You can set it globally in Steam -> Settings -> Compatibility.

By default, games use the global Proton version. If you want a game to use a different version, you can set it in the game’s Properties -> Compatibility by checking the “Force the use […]” checkbox, then selecting one from the menu.

Regressions can and have happened. Steam offers the latest release of every major version, so if a game needs an older version, you can use it.

You can also use third-party compatibility tools that offer different game-specific fixes. The most popular ones are GE-Proton, Proton-EM, and Dawn Winery (the last one specifically for Chinese gacha games with horrid anti-cheat). You can install them by extracting the archives into the ~/.local/share/Steam/compatibilitytools.d directory.

I strongly recommend consulting ProtonDB if you’re having issues with a game.