Well just to get it clear. UDM and FirtzBox are the Wireguard-Servers, not any other machine in the networks behind?
If so its not diffrent to what i do with my pfsense and a remote FritzBox.
The major point is FritzBox does not like by defalult transfer networks in Wireguard-Connections.
I have no idea whether or how well you understand German but here on this Site are instructions how to setup Wiregurad between FritzBox and pfsense without using a transfer network. But its possible to add it later if all works.
#Anleitung: Site2Site WireGuard Verbindung zwischen pfSense und FritzBox
Maybe you shall start with that