You can set both the wireguard and eoip mtu’s to 1500, it becomes less efficient as the larger packets are fragmented, but they get
rebuilt at the endpoint. Perhaps set the eoip mtu to 1500 and leave the wireguard one at 1420 (1420 assumes no pppoe).
An alternative, you can use a mangle rule to do mss clamping (clamp to pmtu), but only good for tcp/ip.