You’d be far more secure using the Flint as a WG Server then connecting to the SSH daemon (dropbear) over that. Just be sure to set a WG preshared key (PSK). You won’t have to fuss with port forwarding or other firewall changes either. GL GUI handles it all for you.
Login to LuCI with the userid of root and the same Admin Password you use to login to the GL.iNet Admin Panel.
To add an SSH-key, go to System > Administration and select the SSH-Keys tab. Paste in the contents of your id_rsa.pub file and then click the Add key button. Its probably best for you to try to login to your router from your LAN on port 22 using your SSH-key and make sure everything works before proceding ('cause otherwise you may end up locking yourself out).
Select System > Administration. Select the SSH Access tab.
To change the port, just key your new port number.
To open SSH up to the WAN, use the dropdown box to select unspecified (this will allow ssh access via LAN, WLAN, and WAN).
To require only SSH-keys, uncheck the Allow root logins with password and also uncheck the Password authentication checkbox.
Afer you’ve made these changes, click the Save & Apply button. Note: you can make the above changes one at a time, clicking Save & Apply, and then testing that each step works as expected. That last step is kinda final - once you nolonger allow passwords, you can only access the router with a key; if something goes wrong, you’ll end up having to reset the router which can be a PITA.