Mine is Local Send which is a FOSS alternative similar to air drop that works across a variety of devices.
No spam, because it is a family group for sharing non-public pictures etc.
You’d only get spam if you invited a spammer to chat.
The privacy comes from the E2EE.
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.]
Do note, because it’s using email, the recipient and sender are not private, along with the time, and probably the relative size of the messages.
The specific content of each message should be private as long as the encryption is done well. I haven’t looked at it so I don’t know if it implemnts safeguards to verify who you’re messaging with (besides using the email address) and I don’t know if it uses PFS (Perfect Forward Secrecy) to protect against a key getting compromised.
