Already have ZigBee and already bought a temp/humidity sensor from Sonoff.

If you have Zigbee poke around and see if the Aqara sensors will work with google home. I have a half dozen running in our house and they work great-- I can get temp/humidity by location via voice. With Alexa. So I haven't bothered to even try them with our Google home gear, which frankly seems to perform worse every week so I've mostly relegated it to simply playing music and setting timers.