Thanks @claude0001 and to the whole forum. I went from stock Android to Ubuntu Touch and back, without rendering my device (even more) useless.
And while Ubuntu on a phone is nice, it really is far less powerful then I have imagined. No Firefox, no Thunderbird, ages away from running docker ... it sure works, it's also actually nice, but by no means could that ever be my daily driver in that setup.
Still, as I waited almost 4 years for this device, I wanted to give it a try. At least I squeezed out the guide, hoping this is helpful for another lost Linux-Phone person out there.