hello @ArmyCore
Sometimes disabling STP on a managed switch isn't enough – some switches implement that as "disallow STP entirely" which definitely won't work in this case (we need STP to be running, we just need the switch to pass it through and not try to be the root of the spanning tree). As I said earlier, I know people have had particular trouble making managed Cisco switches work. In any case, replacing it with an unmanaged switch should help significantly.
Google WiFi should be able to manage routing for 1Gbps internet service, which is the fastest its Ethernet ports can support anyway. The Nest WiFi Router is built on a newer processor, but it's hard to compare them since they are very different in terms of instructions per clock, clock frequency, and total core count. Still, the Nest WiFi Router should be able to handle 1Gbps routing duties as well. I doubt it would be a huge improvement over Google WiFi on this score, though. You would have to factory reset everything and create a new mesh network to replace the primary/router unit (though you can reuse the same SSID/network name and password so that all existing clients reconnect automatically).
The Nest WiFi Pro is similar from a software perspective and has a comparably powerful processor. It also has 1Gbps Ethernet ports. The big difference is that it supports three WiFi bands instead of only two: 2.4GHz, 5GHz and 6GHz. In the Google/Nest WiFi systems, the 5GHz radios are used to talk to 5GHz WiFi clients and to carry the 802.11s wireless mesh interconnect. In your case, since you have wired backhaul for your secondaries, this interconnect is essentially idle, so your 5GHz clients can get all of that channel capacity. In the Nest WiFi Pro system, the 802.11s mesh interconnect runs in the 6GHz band instead, leaving the 2.4GHz and 5GHz bands available for client traffic, while 6GHz clients share that channel with the mesh interconnect. Again, though, if you are wiring up the secondaries, this 6GHz channel would be essentially 100% available to 6GHz clients (and there are some, but not a lot of those today). The most important thing to know about Nest WiFi Pro, though, is that it is not compatible with any of your existing Google/Nest WiFi units. You can't replace just the primary/router unit with a Nest WiFi Pro and keep using your existing Google/Nest WiFi hardware with it. You'd have to replace everything with Nest WiFi Pro units.
I guess my advice would be to focus on optimizing the system you have now, then run more detailed tests to help understand where any bottlenecks may be, and think about how to improve those if necessary (and it may not be necessary – there will always be a bottleneck somewhere – you just want that to be in the internet service rather than inside your network if possible).
