If all you want is Wireguard you’d almost certainly be fine with a 2GB. I run Wireguard on bare metal and 6 or 7 containers on a 4GB Pi and I’m only using half my RAM
Setup Ubuntu on the 8gb and use it as a docker host OP
Works great! Plus room for growth. And you can’t upgrade it later
No need for a docker host, I have a dedicated server.