Can't resolve "No space left on device" error message

> Try Safe Mode, as per > Use safe mode on your Mac - Apple Support

I've already tried safe mode a few times (also when trying to boot my extra Mojave partition). It always fails to boot.

> If it doesn't startup Safe Mode. Startup in Recovery Mode > Disk Utility > Erase > then Reinstall and Migrate from your Time Machine backup.

Yeah, I've pretty much already come to that conclusion. Was hoping there might be some more I could try first.

One question I have is exactly what should I erase. The top of the disk hierarchy is "Fusion Drive", which it doesn't appear I can erase. Within it is "Container disk4", which I can erase, and within "Container disk4" are my 3 volumes, each of which can be erased. The inability to choose a startup disk (see my "SDErrorDomain error 104" comment below") has me thinking if I just to erase a volume, I might still have issue.

A couple of other things to point out. I have a couple of other partitions in the same container. One of them is an old clean install of Mojave (meant for emergency purposes, but unused in years). If I try to boot into it, it fails with:

"Your computer restarted because of a problem. Press a key or wait a few seconds to continue starting up."

I was hoping this partition would work even though it shares the container with the full partition. It does have a about 92gb of reserved memory free that the other partitions can't grab. I'm thinking that maybe I should delete this partition to free up some space, although I worry that this snapshot issue is just going to eventually eat it up to.

Also, I tried using Startup Disk from recovery mode to to change the startup disk to this Mojave partition, but when I do it says "Unable to set startup disk", with the error being SDErrorDomain error 104. So instead I just hold the option key down on boot and select the partition.

> Also consider moving your Photos Library, Music Library or Movies Library to an External Hard Drive.

Please re-read my post. I can't even boot, so this is not possible. Even the Terminal app from recovery mode cannot delete anything on this drive. "rm <filename>" results in "No space left on device". This is about as low level a file deletion process as you can get, and it fails.

BTW, "erase" of a volume fails with:

"An internal error has occurred.: (-69624)"

Seems this is a somewhat common issue that I have yet to see an explanation or solution for. Note, the partition no longer exists after doing this. This resulted in the previously full partitions now have about 12gb free, yet any attempt to delete a file or snapshot still results in "No space left on device"