How would you fix this on a (physical) Windows machine when the disk was full and it does not boot anymore?
Maybe boot the VM with a GParted Live ISO and hope it can mount the virtual disk to delete files? Does Windows have rescue boot options. maybe?
Maybe add another Proxmox storage (remote NFS/SMB or physical drive) and create another Windows VM and use that to mount the existing drive to delete files?
I would restore a backup but you seem to have excluded the virtual disks. Since you don't seem to care about the data.I would delete the VM and make a new one (but not use thin storage).