The drive showing > 100% usage with UFS is not that unusual as UFS has some "extra" space it holds, it just means that it's really full when it hits around what you see there.
Something you have must be holding open a log file somewhere that's consuming the space. If it goes away on reboot then it may be files in /tmp as well. You'd have to inspect the contents of the filesystem when it's close to full to see what is using the space.
https://docs.netgate.com/pfsense/en/latest/troubleshooting/filesystem-usage.html
It's also possible the filesystem has some slight corruption that is making the numbers look off from reality, and running a manual filesystem check may help:
https://docs.netgate.com/pfsense/en/latest/troubleshooting/filesystem-check.html
If/when you do reinstall, you're probably better off moving to ZFS, but that wouldn't help if something is actually filling up the disk.