I always saw Zerotier as a very nice implemenation for STUN/TURN/ICE , where Zerotier has a free public TURN server, and will switch to STUN if possible. If not possible to switch to STUN it will continue on TURN (passing the traffic over the TURN server). STUN goes direct between the 2 involved sites without going over the TURN server, if the “UDP hole punching” or other connection spoofing techniques do work.
https://www.zerotier.com/2014/08/25/the-state-of-nat-traversal/
Maybe there is somewhere a STUN/TURN/ICE server or setup that can be used for Wireguard without access via public IP on all sites with MT.
https://medium.com/orion-innovation-turkey/webrtc-in-a-nutshell-ep-iii-19ce0e967969