Oh don't :)

I'm actually worried that some newer smart devices might be set up to use well known public wifi services that are available from consumer routers.

Just sitting here I have public "EE Wifi X" and "BT Internet" which it could connect to if configured at the factory to do that.

Then I am boned.

Check out Amazon Sidewalk—if any of your neighbors connect a device supporting it to the internet, your devices will find them and use their internet connection if you don’t provide one! Apparently their network covers like 90% of the US population (at a low data rate), and they’ve made it possible for non-Amazon devices to implement it.

Ok so my next house: must have Faraday cage.