Get also a problem with another wireguard connection to another network (here with OPNsense WG-server).
It seems to have no matter what MTU I set on the client. If I connect, I can ping exactly for times with very bad speed (about 200ms) and then connection dies. If I just reconnect, I can send again 4 pings.
not sure, if it has something to do with the OPNsense
Edit:
false key setting was the problem on the laptop and not MTU: