Hi @ebon tapir , your question makes me think, and yes, what I talked about would lead to having your eve plug more directly connected to the server network than to the client network. And I was believing that this is what you actually wanted: When you said in the beginning that your HA resides in the server network I made two assumptions:
a. That was an explicit decision you made, and I took it as a constraint
b. You were already having other IoT devices (directly) controlled by your Home Assistant
So I drew the conclusion that you wanted your IoT devices to live in/near the server network. I didn't ask though -- my bad.
And yes, what @blissful adder says is also true, and this is why my idea was to put the border router(s) into the server network as well: Because that is the easiest way to ensure HA and the border router(s) can talk to each other (assuming you wanted HA in the server network).
But of course, if you actually wanted the IoT devices to be in/near the client network, then putting HA (and the border routers) into the client network would be the most simple setup ensuring connectivity.
The most complicated/complex and in several scenarios not so reliable set-up could be to have border routers and HA in separate networks. I am not saying it's impossible, but a lot also depends on your network gear and IPv6 setup.