Hello @cajunmike1976
First, take a look at this help page for all the details on wiring secondary Google/Nest WiFi Router units: https://support.google.com/wifi/answer/7215624?hl=en
But, it's also important to understand a few other details about how this works. The 802.11s mesh protocol only runs over a (hidden) WiFi connection between the Nest WiFi units. It doesn't run over Ethernet. So, when you connect another Nest WiFi Router unit as a secondary back to the primary Nest WiFi Router's LAN Ethernet port, that secondary Nest WiFi Router has to stop talking to the wireless mesh. Instead, it will carry traffic for any nearby client devices back to the primary over Ethernet directly. This is very nice for performance, and allows you to place that secondary much further away from the primary than might otherwise be practical. But, since it can't talk to both Ethernet and the mesh without causing a "loop" in the network, it also can't act like a "base station" for more distant mesh-only secondaries. This will impact your plans.
Another important thing to understand is that, just focusing on how the 802.11s mesh works alone, the "next hop" selection algorithm is designed to prefer fewer "hops" through the mesh. So, if a mesh secondary can get through to a destination at all, it will talk directly to it – even if that connection is relatively slow (and WiFi will slow way down to get through). There's only one 5GHz radio, so every extra hop means using that radio (and the same channel) multiple times, so multiple hops is often slower than a direct connection anyway (especially if the traffic is for a 5GHz WiFi client, since that's using that same radio and same channel one more time). This is why the placement advice recommends putting the primary as close to the center of your home as possible with wireless mesh secondaries one or two rooms away from the primary (close enough to get a strong 5GHz connection). From there, they will provide both 2.4GHz and 5GHz coverage to more distant clients. The result is a reliable deployment that has a larger coverage area than a single access point could support.
Wired secondaries, as I said, can be placed much further away – even in outbuildings, if the cabling is in place.
So, in your situation, I think I would try to use Ethernet to get your primary closer to the center of the home rather than leaving it at the back. Then place your secondaries around it as recommended. If you can also run Ethernet from there to another Nest WiFi Router unit (or Google WiFi units), that will certainly help take some load off of the 5GHz channel and allow that unit to be placed further away. It just won't be part of the wireless mesh (in any way that matters).
