Many more months have passed, the Jelly Pro is still in my life, and it now runs Android 8.1.
Android 8.1 runs well, but one configuration setting absolutely
must be changed:
settings → smart assistant → power save
manager and then turn that off. The power saving
regime provided by Android 8 is better, and the third-party
save manager Unihertz has included confuses
into thinking that apps are broken when they're working perfectly
well. This is why Signal doesn't receive messages, and it also
prevents apps from e.g. scheduling heavy work to be run while the
phone is charging.
Many reviews of the Jelly Pro describe insufferable battery life. That's true and accurate, but it's also a tiny little bit irrelevant for me since what I want is a phone that can run smartphone apps but I don't want to check the phone every few minutes. I am the master, the phone serves me, and any app that buzzes me needlessly loses its permission to issue notifications. The battery is good enough for my kind of usage.
I do wish the phone would charge faster though.