Follow-up: I added the Guardian project repo to FDroid. Turn out: once FDroid has the repo, it can "take over" and do updates, even if Signal was originally installed from the Play Store / Aurora. That pretty much solved my primary issue here. (I'll look into Molly at a later point anyway, just for the sake of curiosity.)
Thank you, once again!
Fair, that's why I want to avoid it as much as I can.