Not saying that it’s necessarily a bad option, but my biggest issue with delta chat is that it does not offer forward secrecy (if a user’s private key is compromised, past messages can be revealed); Signal does. Delta no question beats signal in decentralization, though email is less decentralized than it seems–how many people do you know who still use gmail? Delta also inherently leaks metadata on whom you’re communicating with to the email host (that’s just imap/smtp). Signal can mitigate this somewhat with Sealed Sender (which gives one-way anonymity), though it can be broken with statistical analysis, and signal metadata is more identifying due to requiring a phone number.

FYI, while Threema front-end clients (apps) are open-source (and offer reproducible builds, which is surprisingly uncommon in open-source land), the server component, though supposedly audited, remains closed-source.

EDIT: for comparison, the Signal server code is mostly open source, but things like the spam filter are closed.