Hello @Paul-76
Just a note that I moved this message to create a new thread in hopes of getting more focused discussion on your questions.
Regarding your first question, the main and guest WiFi networks use the same radios (both 2.4GHz and 5GHz) as each other. They just have different network names and passwords in addition to some other higher level security features. The interference and performance should be exactly the same since they'll still be sharing the same channels. Regarding intermittent performance issues, that can be hard to diagnose since there are so many things that could be contributing to it. I always start with a mesh test to ensure all of the secondary / mesh point units are showing a "great" mesh quality. If any of them aren't move them closer to the primary and re-run the test. Another suggestion is, since you are using Google WiFi now, and those units have Ethernet ports, you can connect her laptop to a nearby Google WiFi secondary via an Ethernet cable to skip one WiFi "hop" in the path. Obviously, this has implications regarding placement and convenience. Anyway, those are just a couple of ideas.
Regarding your second question, The Nest WiFi Router hardware does have a bit better WiFi radio than the Google WiFi hardware, but it may not be better in a way that matters under most conditions. But, if using it frees up a Google WiFi unit to use somewhere that having Ethernet would be helpful (e.g., see above), then it might be worth it. However, be aware that doing this will require performing a complete factory reset of all your existing units, then setting up the Nest WiFi Router unit as a new primary and then adding all of the old units one by one to the new network. You can (and should) use the same network name and WiFi password so at least your clients won't have to be reconfigured, but it will still be a fair amount of work. An alternative would be to add the Nest WiFi Router unit as if it were a secondary / point to your existing network. But, I'm a little confused, since you say you already have a Google WiFi unit in your office – why not just use the Ethernet ports on that unit where it is already? All of the Google WiFi units are the same in terms of hardware – the one that acts as a primary / router is just the first one you set up and connect to your internet service via Ethernet.
Lastly, the Nest Point unit will need to be placed close enough to the primary (whether that's the existing Google WiFi primary / router or the new Nest WiFi Router unit). Sometimes more access points isn't better, and can even be worse.
I hope some of this helps!
