in particular, they should really use the backup provider API provided by PVE (which would also allow them to easily run the agent itself in a VM, if desired).

I've setup this too, on a 3-node cluster.

However, some backups run just fine, others throw strange errors like "Unknown device 'ide0'." even though given VM doesn't have any ide devices attached...
I'm also having trouble restoring a VM from a ESXI backup to Proxmox...

EDIT:
"Unknown device 'ide0'." errors even though no IDE device is connected to VM is caused by Option->boot order.
If there is an IDE device her, remove it.
This will be fixed in later version.

EDIT 2:
Cross platform restore on Prox is not yet supported.
According to Acronis, this will be suported in version 25.08 release.
Until then, one needs to use Acronis bootable media to restore cross platform to Proxmox.

EDIT 3:
Agentless backup from Proxmox is NOT yet supported on PVE9...!