If you want the best setup that does not cost much more then highly recommend instead getting three (or more) Sonoff ZBDongle-P or Sonoff ZBDongle-E adapters to use in USB-chargers for power after converting them into dedicated Zigbee repeaters / range extenders by flashing Zigbee Router firmware.
While IKEA Trådfi Signal Repeater is good (and better as a dedicated Zigbee repeater / range extender than for example the IKEA Trådfri Control Outlet), the CC2652P and EFR32MG21 based ones with external antenna like those Sonoff ZBDongle-P or Sonoff ZBDongle-E adapters work great as dedicated Zigbee repeaters / range extenders, best of the best!
If that does not help then recommend reconsidering changing to other Zigbee channel (as well as taking all the other suggested best practice actions to avoid interference that are listed in my original post).
hmm, you can change the Zigbee channel in ZHA integration without having to re-pair ALL devices. I do not know how Zigbee2MQTT has implemented changing channels, perhaps Z2M developers have not yet implemented broadcasting of channel change which gives the devices a heads-up that the channel will change, (it could also be that it is old or wrong information as it is possible in most Zigbee solutions, and while not all Zigbee devices support changing channels without re-pairing, most devices do).
Anyway, the fact is that most Zigbee Router devices (mains-powered products) should handle switching Zigbee channels and will usually reconnect themselves automatically within 1-hour or so and you can even force that process to go quicker by power-cycling devices. It is normally just the Zigbee End-Point devices (e.g. battery-powered devices) that will be re-paired/re-joined. You should not lose any configuration or data as long as you only re-pair/re-join the device and do not remove the device from your Zigbee gateway solution.
No (that is not how Zigbee is designed to work), but many Zigbee gateway solutions do at least perform an energy scan on the initial setup and pick the least congested channel at the time of creating the network.
You also have to remember that practically almost all commercial Zigbee gateways/bridges/hubs from companies only support a limited set of specific Zigbee devices that they have certified for their Zigbee gateway/bridge/hub, and most only support devices from their own brand, meaning that you can just buy any device with a Zigbee logo on the box and expect it to work.
If using Home Assistant then the only Zigbee solutions I would even consider for a fresh installation are the ZHA integration or Zigbee2MQTT, (and deCONZ/Phoscon also works good if migrating from another ecosystem that used it, however, I would not personally recommend it today for a new installation).
Yep that sounds very plausible. Be sure to connect all your WiFi access-points and other devices to Ethernet if possible. IIRC I am pretty sure that was one of the tips in my original post above
Most such WiFi solutions that assign channels automatically tend to primarily use WiFi channels 1, 6 and 11 so suggest try changing to Zigbee channel 25 (as many devices do not support Zigbee channel 26).
Obviously, you always also want to locate your Zigbee Coordinator and all your Zigbee devices as far away as possible from your WiFi access-points and any devices that use Wi-Fi, regardless of setup.
Here is by the way good article explaining this is Zigbee and Wi-Fi Coexistence → https://support.metageek.com/hc/en-us/articles/203845040-ZigBee-and-WiFi-Coexistence
hehe, you do not have many devices now, so will feel much worse later when you have loads more
If you want to keep your Google Nest Wifi then you got to be willing to work around its limitations.
I have owned a similar WiFi solution myself in the past and learned my lesson of not doing enough research, so sold it before owning it even for 1-year and bought UniFi APs from Ubiquiti, no regrets.