I used to have issues on the original Jolla phone (with is notorious for its BTRFS balancing issues), but I haven't had it on the JollaC. I now use an sd card, and make sure that the camera stores pics in there, and have also moved any docs over. Some general housekeeping like deleting browser cache (and maybe from some android apps too) might help to. Code: du -h --max-depth=1 | grep M will give you the disk usage of each directory in the current location, and is a good starter for tracking down the guilty apps. On my desktop PC a program called Acestream had gigabytes locked up in useless cache files.
du -h --max-depth=1 | grep M