If you have a /24 segmented into several /26's and you want to route between each segment that's the job of the router. I HAVE the router.
The problem here is I cannot tell BG3 to direct connect to an ip within a different segment than the one I'm currently on.
Direct Connect pokes a hole in your firewall w/ reverse nat (I presume) and then anyone connecting to you within your home needs needs to traverse your firewall twice and have that same packet natted twice .. once for outbound and again for inbound.
The opportunity to route doesn't even occur because the destination isn't one on your network.
Really the easiest solution is for multiplayer to simply offer the opportunity to connect to a specific ip. With that, the router can actually route the packets away from the firewall/nat and onto the appropriate lan segment.
Your comment about setting those vlans to share the same network makes no sense.
Instead it seems like you meant to say put all those machines in the same vlan which yes, would work, but defeats the purpose of keeping unstrusted networks segregated from trusted networks.