I should do more.

This helped

sudo apt modernize-sources

We’re all waiting to hear about what Linus decided regarding bcachefs, aren’t we?

Did you disable fastboot in windows to make sure this doesn’t happen again?

Filesystems may be remounted read only on error. I’d expect that to be reverted post reboot, but maybe your error was big enough for it not to? You may want to search how to perform an fsck on the affected partitions.

Random guess: Install flatseal and use it to give permissions to that folder.