Which product would be best to determine why my Nest Hello Doorbell is fluctuating from -60 dBm to -75 dBm?

Hands down the best tool for this would be inSSIDer with a MetaGeek Plus subscription here and a compatible packet capture adapter from this list. With this you would be able to monitor the doorbell’s behavior on your network to see if it’s band steering and at what signal strength. Since the Nest Doorbell probably doesn’t send beamforming reports, it would also be a good idea to place your laptop running inSSIDer next to the doorbell and see what signal strength you are seeing. You may find that the AP is using a band steering mechanism to encourage one band over the other, which may be confusing for a client device like a Nest Doorbell. If that is the case, you may want to disable any band-steering protocol from the AP side or simply give each band its own SSID name, as discussed in this video.