I neglected to mention the most annoying blocker for me: Deltachat is not in the Debian package database.
I recently tried to install Deltatouch on Mobian (mobile Debian), and all I managed was a blank icon in the apps tray. and I made a stupid permissions error which broke the install, and still more stupidly, actually needed help from the dev, lk108, to sort it out. I could try installing Deltachat-desktop using Flatpak, but I don’t think that the friend’s phone I also want to install it on has the space.
Mobian does not currently have a native chat client with a UI that works on a small touch screen. If Deltachat were in Debian, it would almost certainly become part of the default Mobian install. There is therefore a window of opportunity for Mobian to drive Deltachat adoption (and vice-versa).
If the Delta Chat team could talk to the Debian Rust Team and the Mobian team and work out what it would take to get Deltachat on Debian, that would be a step towards eliminating the missing feature I find most annoying.
Edited to make it clear that this is not a negative reflection on Deltatouch, which is now installed and working well, and which depends on only one non-Debian package besides the Deltachat core.