FWIW, you could also add some tooling that uses the Proxmox Backup Server periodically saves the list of created backups in some directory, or database of your own.
Even using the debug tooling we provide could be enough, e.g...
IMHO it's better to use PBS sync to copy backups from one PBS to another instead of doing backup of the VM multiple times, even if PVE could use multiple dirty-maps for a few PBS. Less load on source PVE, less load on VMs and you offload the...