Currently, the schematic and board layouts are done.
Boards are already ordered and will arrive in a few days time.

The mechanical design of the cartridge is still a WIP, but should be able to fit the expansion port. As the C64 edge connector used is not keyed, notches on the side prevent accidentally plugging it the wrong way.

Currently, I've made two cartridges to go with it: an Raspberry Pi compatible pinout, and a passthrough.
The RPi interface cart can handle all but SPI1, but features all plethora of other options with the Teensy.