I got it working! It was actually not a big deal. Because Android 13 included Google’s “Project Treble” work, I was able to just drop a GSI build and call it a day. Here’s how:

$ adb reboot bootloader     # Wait for the "Unihertz" logo, there's no other prompt
$ fastboot flashing unlock  # Press volume up within 5 seconds
$ fastboot reboot fastboot  # Wait for "fastbootd" menu
$ fastboot erase system_a
$ fastboot flash system_a lineage-20.0-20250606-UNOFFICIAL-arm64_bvN.img
$ fastboot --set-active=a
$ fastboot erase userdata
$ fastboot reboot

That was all there was to it!

Installing APKs

A few things aren’t in F-Droid (such as F-Droid itself).

$ sideload () { adb push $1 /data/local/tmp/$1; adb shell pm install -r /data/local/tmp/$1; adb shell rm /data/local/tmp/$1; }

Where to get stuff (November 2025):

• F-Droid app store
• Improved-Twelve music player with Jellyfin support
• Signal secure instant messaging and voice/video chat

Handy things to know

img names

The last part of the .img filename tells you how it’s set up:

• bvN means “a/b booting, vanilla android, not rooted”
• bvS means “a/b booting, vanilla android, rooted”
• bgN means “a/b booting, google apps, not rooted”

… and so on. You always want the a/b booting version. The rest is up to the installer to decide. I went with vanilla, not rooted.

Lineage 21 didn’t work

Lineage-21.0-20250621 won’t boot for me. I had to use 20.

Getting into fastbootd without adb

Hold vol+ during boot to enter into “dead android” mode, it will say “no command”. Then press Power and Vol+ briefly to get into android recovery. From there, you can enter fastboot directly.