In my case it’s that hard separation between devices. I like reading on the eink screen compared to the phone screen, and there are no notifications to interrupt me. It helps too that the ereader is just ready to use, no decision to fight about what app to open. I tightly manage notifications only phone and it’s still too much of a draw to go doomscroll instead of read. I frequently pull just the reader out and leave the phone so I just use the magnetic attachment to keep them together.

This sounds like you’ll need to do a balance operation. Try this first and see if it helps:

btrfs balance start -dusage=0 -musage=0 /

If not you can increase the number to 5 or 10. This operation reallocates chunks on the disk and ensures they’re filled - check https://btrfs.readthedocs.io/en/latest/Balance.html for details.