Hi.. I'm currently back on macOS but ran Ubuntu 20.04 on my iMac for ~4 months exclusively. As in no macOS install at all. Purely Ubuntu. At least once a month, my boot disk would revert to read only.

GRUB: it's neither larva, fast food nor the loveliest of acronyms in the GNU herd of free software. Rather, GRUB is the GNU GRand Unified Bootloader. And, it is truly the greatest loader for booting ...

Something went wrong during the install causing GRUB not to be properly configured it sounds like. You could try fixing it, but I'm no expert honestly. I'd just ...