Virtual network won't start because iptables-nf complains about nat table being incompatible.

Change your nftables.conf (or whatever) rules to call the table something other than 'nat' i.e. VM_RULES or something.

Certainly an issue I ran into on Debian 11.

Also (I was forwarding from public IPs to VMs) I needed to start nftables AGAIN AFTER libvirtd has messed about. 

Actually the proper way to do any of this might be with /etc/libvirt/hooks/network and then load a set of specific nftables rules that way like:-

/etc/libvirt/hooks/network

#!/bin/bash
if [ "$2" = "started" ]; then
/usr/sbin/nft -f /etc/nftables-VM-forwards.conf
fi