First step there would be to get some logs. Launch steam from a terminal and output to a file:

steam 2>&1 | tee steam.log

Launch a game and watch the terminal to see what happens.

If you moved games after launching them, clearing their Proton containers can help. I had some games set to extremely old Proton versions that couldn't launch because the version jump was too big. I've also seen vanilla wine prefixes break when moved due to some symlinks.

But those are guesses, the logs will tell you. Sometimes it even actually tells you how to fix it!

If something doesn't work on one drive and then magically works when you install it on another drive...

How is your drive formatted? Is it NTFS and not keeping the right Linux file permissions or something like that? It's been a decade since I had a Windows formatted drive, but I seem to remember there were issues using NTFS in Linux.

Just a thought, does the path to the games on your OS drive have a space anywhere in it? I remember a lot of old Windows games would throw a shit fit if they had a space anywhere

You 100% can move the folder. Copy it to whatever drive and symlink it in the original location.

You installed Nvidia drivers?

Since that 545 drivers, I cannot play with Proton 8 at all. Open source drivers don't have this problem (their problem is performance), so it seems Nvidia f*cked up as usual. However first time hearing this. Can you play native games at least?

Can you check which GPU APIs are being used by your games? (ex. OpenGL, Vulkan) Are those games 64-bit, or 32-bit?

Additionally, you're not missing out on VR side. SteamVR is horrendously broken on Linux