Slightly off-topic, however, from a design inspiration and concept only point-of-view it maybe be a good idea to look at how a XMOS XTAG debug pad and external IO pins can be exposed on this new upcoming ReSpeaker round circuit board that Seeed Studio has announced for pre-sale:
While this board will obviously not fit the Google Nest Mini or Google Home Mini just beause it is round, I do however believe it shows a concept where there could perhaps be possible to feature both a 4-mic circular array as well as some debug/GPIO pads for the XMOS chip for XTAG-4 debug adapter (XMOS JTAG debugging) on a somewhat similar round board.
The point being it that firmware loading and debug access should preferably alays be possible via an XMOS standard XSYS2 connector which supports the XTAG4 debug adapter on these boards:
Technical specification summery posted at cnx-software:
Note that this new board from Seeed will be based on the XMOS XVF3800 chip instead of the XU316 chip, which might be better suited for voice capture and smart speaker use cases if it also supports open-source firmware?
PS: For discussion about ReSpeaker XMOS XVF3800 ESPHome integration see this thread: