I’m strongly considering adding another backup location in the form of an old Raspberry Pi and a USB HDD.
I want the Pi to exclusively use the available network to connect to my Wireguard Server, so other devices (local to the Wireguard Server and remote connected to the server) can use it as a secondary backup location.
I’m kind of worried about a scenario, where my network is compromised and, through the VPN connection of the Pi in the external network, the external network is as well.
What are the best practices to secure such a setup?
Go the other way. Have the backup server connect in to the rest of the network.
I was considering doing something similar to OP, but I also think it’s better to do it the other way around, having the backup server connect to the network when it’s time to do a backup. Then you can just use the trusty ssh/rsync combo for backup.