This is just my opinion. I think its uncontroversial, tho ...
Proxmox Backup Server doesn't back up physical hosts well. That includes PBS itself.
There is certainly a tool for it and a whole wiki page to help you through.
https://pbs.proxmox.com/docs/backup-client.html
The problem with every method I've seen for backing up PBS is that they are file-level backups.
In order to recover, you need a booted copy of ... I guess ... Debian to overwrite with all the data.
How you gonna do that? Well there are ways ...
Some of the forum members have come up with their own methods.
A couple of folks use Relax and Recover (ReaR). I'm not smart enough to get ReaR working.
Neobin wrote his own script, its on github, I lost the link.
My own solution is simply to document PBS config and treat it like an appliance.
Don't configure anything I can't rebuild.