This might not be your issue, but it might be useful to know: if you connect two disks with the same GPT UUID, they might start misbehaving. Perhaps GRUB might not care, but Linux does. Did you ever clone a disk or partition without changing the UUID?

Can I do a shameless plug? Not necessarily all of these are my favourites, but most of these are pretty nice. Here’s my list.

My top fav is Mindustry, which has already been mentioned. Another not very popular FOSS game which is pretty good imho is OpenClonk.