There are some threads here in the forum about that. The major things are the
Guests are best backed up with the integrated backup. This will enable you to restore them to any other PVE server even if it does have a different storage configuration.
When restoring these configs, don't restore them blindly but rather take them as a guide of how things were.
/etc/pve directory and the /etc/network/interfaces file. Whatever else you might have configured (PCI passthrough, ZFS ARC (cache) limit etc) you will have to decide if you back that up as well.Guests are best backed up with the integrated backup. This will enable you to restore them to any other PVE server even if it does have a different storage configuration.
When restoring these configs, don't restore them blindly but rather take them as a guide of how things were.