Yeah, it seems your Ethernet is the priority connection (GL GUI -> Network -> Multi-WAN -> Interface Priority).
My first instinct is to use some custom routing to send your WG server traffic out over $RepeaterIP but the immed. problem w/ that is that, well, your $RepeaterIP changes when you change the upstream/its Wi-Fi connection. I'm not convinced it's the most efficient solution.
Regardless I foresee you're going to need to get into LuCI & SSH. If you haven't set that up already, see the attached HOW-TO; it'll walk you through a practical exercise for just that.
I'll hold back a bit for others who may have better insights; I have a tendency to look to 'roll my own' solution than use stock tools.