There is this but it's replacing almost everything inside. https://hackaday.com/2023/07/23/google-nest-mini-gutted-and-rebuilt-to-run-custom-agents/ I am hoping they sell the boards but most likely won't out of fear of Google lawyers.

A year or two back, I think I looked for information on them, and found a few people who disassembled them..... but, the hardware was pretty custom and specific for the application.

That being said, One of these days, I'll take apart one of the ones I have laying around here, and see if it might be possible to solder a esp32 somewhere on the board, to take control over some of the functionality.