I’m throwing in the towel on this project as I’ve had no luck tracking down the firmware.
I did get my hands on another mirror and made a full backup to work with, but have to learn how to do it over ADB as I can’t even get it to boot into recovery. I’ll play with and learn from my mistakes.
In the meantime...
I just ordered a 2 din android head unit and some fun peripherals for it (usb dash cam, OBD2 dongle, Apple CarPlay adapter, etc) from Shenzhen for under $50 total
They should be here in a month or two.
I love my stereo setup and I’m sure the sound on these units is crap, so I plan to keep my current HU and install the Android unit in the lower console. I’ll then 3D print some type of thin storage slot that will fit under it (big enough for a phone, where the batwing would be normally, maybe even an actual dock).
The unit I purchased is very plain on the front (with no volume knob), so it shouldn’t look too out of place with the current stereo.