I turned on discard and ran fstrim in Ubuntu (since that's where trim is needed in my case -> on the actual VM) and proxmox once again reports 10TB free, spa_slop_shift is back to 5 and things are running great!
Thanks a lot for the help and pointers here I thought I was completely screwed!
So...
After some digging I set the discard flag to no and managed to boot the VM temporarily with spa_slop_shift set to 5.
VM actually seems fine, which is great.
I removed a lot of data from the VM OS hoping that Proxmox would pick up on it but that didn't really help.
Setting spa_slop_shift back...
Hey guys, would love some help getting back on my feet without being ruined here! :)
I did a large filesync to one of my VMs the other day and it died with IO error.
I can no longer boot it or mount the disk because it's overprovisioned (yes I know, my bad).
The only solution I can think of...