The current state of support for Google Nest products in HA is not the most user friendly and does not support all Nest devices. I have been using a starling hub to integrate with HomeKit for over a year and it has been rock solid. I tried using the HA HomeKit integration mode but this would break the pairing to the Apple Hub and created a number of unwanted devices.
Support in HA for Starling Hub would solve a number of these problems and it has a documented local API.
Also when I contacted them they seemed open to providing test hardware to developers wanting to develop a HA integration. Unfortunately I lack the development skills to create an integration, but would be more than happy to help with testing.
FWIW, I was having a heck of time trying to get Nest thermostats shared via Matter into Hubitat.
I rec’d the HA Green while this was going on, and after installing it eventually HA discovered the Starling Hub device through the HomeKit Device configuration. Honestly, I was at that task for 4 days, reconfiguring/resetting, adding/deleting fabrics. . . I wouldn’t know how to install Starling through the HK Device, it just showed up as “found another thing”.
I now have access to the thermostats through HK and HA w/o Matter. Who knew? [g]