For what its worth I have used the open source Tinc VPN [1] for mesh multihop routing for ages. It is nowhere near as fast as Wireguard but I could envision Tinc incorporating support for Wireguard if the author were so inclined. Like you mentioned Tinc does not mesh directly with other VPN's AFAIK. I've had to use route statements to join it with Strongswan and other VPN networks.

[1] - https://tinc-vpn.org/