Yeap, I thought Axolotl was promising, and it actually interacts with Signal servers with no issues (calls not encrypted so far), however AFAIK it only works as a secondary device, not the main one (I might be mistaken, but I understood that from another lemmy post). I could live with only a Matrix client (unfortunately only Element.io is as featrure complete) and a Signal/Axolotl client, on a pinephone or similar gnu/linux phone, but it seems not possible, and even a Matrix client for a gnu/linux phone is not that clear to me (the gnu/linux clients are desktop oriented ones).

At any rate, I was hoping one could replace Signal with Axolotl, since it does hook to the Signal servers, and interact with them, but I guess I was just hoping...

BTW, matrix alone, won't allow me to connect with any but only 3 contacts of mine (whom I personally installed Element.io for on their phones and computers), but NO one else unfortunately, so I guess there needs to be a trade off, and Signal might be the one feature rich enough, and definitely safer and more private than Whatsapp. Other alternatives oriented to security and privacy might be valid as well, but I don't see them as adopted for a trade off, neither as feature rich. I'm still hopeful a truly decentralized, totally FLOSS, feature rich and easy to use and adopt will get main stream. So far it's having something (still far from perfect) for just a couple of contacts, and a trade off for the rest you still want to keep in touch.

You can totally build signal from source yourself or just use their verifiable builds.