Hi! Thank you so much for trying to help; I really appreciate it.
Basically, I checked and confirmed that my iCloud storage is full (200GB/200GB), which also matches the time frame when the issue started appearing (full a week before). My "Desktop & Documents Folder"-option was already unselected, but I hadn’t checked off "Optimise Mac Storage" until I followed your guide, and I have now done so. My MacBook storage lies at 379/500GB, so I have about 120GB of free space.
However, despite multiple restarts and attempts with my Sims 4 game, the issue still persists despite that fix.
(Transparently, I'm unsure what you meant by step 6 of your guide, as my folder "Electronic Arts" stayed in the Documents folder where it's supposed to be without being removed, so that I might have done something wrong, I'm not sure.)