I can't speak about pfsense or router-based configs, I have zero experience there. The Proton website does have openVPN and wire guard manual setup instructions though. You could try it with a free account first to make sure everything works before committing to paying for it. If you're interested, I can show you the changes I made to the openVPN config file to allow split tunneling. Again though, I don't know how that translates to pfsense.