There's normative open source android firmware, I have a gift phone
   that doesn't work yet, and I have a little experience debugging
   firmware now from coreboot. Just a little bit.
   I'm thinking if it doesn't work, I can in the worst case copy some
   assembly from the working firmware, to maybe provide a usb device or
   light an led, and possibly bisect problems to troubleshoot. If I can
   keep it up. We'll see.
   The unihertz thread shows ValdikSS got treble_experimentations to work
   from a release on January 18th, 2021. There's a strange inconsistency,
   unfortunately: the instructions say twrp can be used, but the twrp post
   is later that year. Still, it makes sense to try the last release prior
   to Jan 18, 2021.