If you haven’t already check ProtonDB for compatibility on your steam library! Anti cheat is usually a no go but some games will work. Non steam games you’ll have to check something like Lutris/Heroic for support.
Someone correct me if I’m wrong but if you’re using stock Pop_OS it uses xorg by default as the compositor which iirc has issues with multiple monitors on different refresh rates. You can edit your .config file to enable the Wayland compositor and give it a try and see if it works!
Playing on a 240hz monitor at 240hz or is it at the default 60hz? Could be as simple as a click of a button!