I changed the Brume 2 Wireguard server to uses Port 51821 to get cascading WireGuard server with NordVPN WireGuard, which had a conflict on Port 51820.
Performance is pretty good (my Internet plan is max. 30Mbps upload from the ISP).
BTW, cascading WireGuard server with NordVPN OpenVPN and cascading OpenVPN server with NordVPN WireGuard both work fine.