While I'm still not certain the goldenshine/Expression V2 board is an official Cricut offering, I went on to acquire a "bricked" AIR and explore and a mainboard. The MCU on later models appears to be aV850ES/Fx3 MCU provided by Microchip Technology https://www.renesas.com/us/en/document/mah/v850esfx3-user-manual-hardware?language=en&r=1055686, which means we could absolutely write some firmware for it, but looking at it, we have only have a few points to interface with the hardware.

The motors in these are DC servos with encoders driven by an Hbridge chip that's not complex. 

The Bluetooth is provided by a relatively normal bridge, also by Microchip Technology, which can be found here: https://fccid.io/T9J-RN42

This might be fun to code for even if I can't take over the V850.

It's not clear to me yet how much code lives on the V850, what lives in the bluetooth bridge, and how much is in the cloud.

There's a ten pin header with cable that looks just like the old keyboard, and a ribbon cable. If they're carrying on the same design, the ribbon activates/controls pressure on the cutting head (which is odd). I won't know where the cable goes until I open up one of mine.