Agree with you, optionally displaying a qr-code for a wireguard client will be a nice thing. dd-wrt does it by default, *very useful*.
And for the firewall rules, set lan->wireguard and wireguard->lan both to Accept/Accept/Accept for testing at least, you may restrict the flows after you are done.
Does your device have a public IPv4 or IPv6 address ? If not it won't be reachable from the outside world.
Regards,