

Update: I managed to create the symlink with the entire steamapps folder, to the point that Steam recognized the game as installed in /home while it was actually on /mnt. However, it failed to launch in the same way as it did when the symlink was not there.
I couldn’t fix it, and also wasted a lot of time on it. In the end, I decided to run everything that works out of the box from the secondary, and those that give problems I move them to the primary drive. That way everything works. Ideally i’d have just all of them in one drive, but it’s just not worth the effort to find out and fix each of them, it’s some kind of nightmare. In the end, it’s just a game location.
For those interested, from 13 games installed, 4 did not work in the secondary drive and 9 do. So it’s not too bad of a %.