Delta Chat is deltachat-core-rust, unless you want to use the ancient c version.
deltachat-core-rust has reproducible builds with nix and there are some projects for auditing rust dependencies (https://rustsec.org and GitHub - crev-dev/cargo-crev: A cryptographically verifiable code review system for the cargo (Rust) package manager.).
The rustc version available in Debian might be too old to build core, you’d need to check.
Also I doubt that Debian maintainers read and audit every line of code that is in the Debian repositories. Anyways in case you find something bad in dc-core dependencies please let us know immediately.
Suppose I’m a bit paranoid/worried about evil/bad dependencies: The bigger risk could be with delta chat-desktop (based on electron) because it uses npm dependencies, while we try to reduce npm dependencies, there are still 600 of them, and I have maybe reviewed 1% myself so far.
So text based clients, there is a python package deltachat-cursed/README.md at master · adbenitez/deltachat-cursed · GitHub but it doesn’t have all features. if you want to use qt there is deltatouch (possibly complicated to build because it is made for ubuntu touch, and can also be build for nixOS, but probably not easy on debian) or kdelachat (based on qt, but very early in development).
If you want to contribute to an existing client or want to make your own, we are happy to answer any questions that you might have. The process is simple, link to the cffi interface of deltachat code (libdeltachat library) or talk to core as a separate process via jsonrpc over stdio (deltachat-rpc-server binary) and if you want to use core there is the third option of using deltachat core as a statically linked rust dependency. + there are also wrapper libraries for python, go, nodejs/javascript.
Making a client is not that hard as this blog post shows, @lk108 started deltatouch in his spare-time: Introducing DeltaTouch, a new community created client for Ubuntu Touch - Delta Chat
Related:
List of all known clients: List of all know Client Projects