I haven’t used Signal.
Is your ‘registered Signal number’ your phone handset number?
From this page:
https://support.signal.org/hc/en-us/articles/360007318691-Register-a-phone-number
[You are right, Delta Chat uses AutoCrypt, which is OpenPGP based.]
I mainly want personal family photos away from long corporate data retention (and possible leaks) and away from AI scrapers.
Delta Chat covers this.
I’ll check out Signal again though, thanks for the recommendation.