iMac won't recognize free internal disk space; operating poorly (barely)

My iMac isn't recognizing the free internal disk space currently available on a 1TB internal drive and [it seems like] is operating poorly as a result. The 'Storage' tab says I've got 589GB of space available (which should be the accurate amount) while Disk Utility says I've got 41GB available (inaccurate). I started to notice this issue when Mail wouldn't open because it said there wasn't enough space on the home drive. Then other programs had trouble starting, outright quitting or issuing fatal bootstrap errors. After a restart, often Finder won't launch (or relaunch).

Steps I've tried to correct the issue:

• a lot of restarts/shutdown, sometimes stalling and requiring a hard boot

• deleting files or moving them to external drives

• First Aid from recovery mode to check the disk for errors, no errors

• Tried to boot into Safe Mode (but no variation of keys seem to get it there)

• Tried to reinstall OS (says not enough free disk space)

• Login with dummy admin, sometimes Finder won't launch in that account either or lagging persists

The problem seems to be exacerbated when there's a non-Time Machine external drive connected on boot up, but it doesn't seem to be consistent enough to point a finger at. I'll attach a screenshot of the OS and Disk Utility windows with drive space I took last night. I rebooted again this morning and the free space in both of those windows has decreased by an additional 6GB having done nothing except restarting the computer.