The Beryl WG Client connection will be manually toggled in the GL GUI → VPN → WG Client when you want to use either/or, correct?
I haven’t used any FRITZ!Box equipment so this could well be hearsay but IIRC their implementation is an adulterated version of WG. It doesn’t/didn’t play nice with ‘pure’ WG, which is what the Beryl uses.
Ah! Found it:
There’s a related link to a potential solution that might work, too.
Re: Burme 2 ↔ FRITZ!Box 4060
I think you’re looking for a S2S between those two WG peers/endpoints. See the following HOW-TO base on using two GL devices. The steps should be similar, sans GUIs, in your case: