I understand that on a single router if Multi-WAN Load Balancing is enabled the WireGuard client will only connect through one of the WAN connections; however if one router (Spitz AX) was setup for Multi-WAN Load Balancing and a second router (Slate) was placed behind it as a VPN client, would you achieve the benefits of the Load Balancing? Or would the traffic still be send through only one of the WAN connections?
I don’t think the Spitz AX is going to care about the encrypted packets flowing downstream from the Slate.
Force Refresh Streams might be what you’re looking for during testing as a Failover before deploying as Load Balance (GL GUI → Network → Multi-WAN). You should be able to simulate a ‘downed’ interface during Failover just unplugging one of the WAN links.