Thanks for the pointing of XMPP / OMEMO @olf.
Unfortunately, though I joined a Jabber server a long time ago, it quickly became not-so-useful to reach my contacts (when GTalk and other proprietary messengers jumped ship) and I didn’t even experience the Sailfish integration - but understand it is not OMEMO if I recall correctly…
Also, I mentioned Matrix because I kind of use it, but I did not imagine it being a P2P chat app, but more like communities that were once driven on IRC (and today Slack or Discord and others joined). I will take it more seriously for person to person chat from now on indeed! Considering there are at least two ways of reaching Matrix 1 (Sailtrix and Hydrogen Web) this is indeed a good place to be.
(And instead of a later edit to my previous “rant”: I just want to say that I am not a researcher and only wrote the above with my gut feeling. Some people here may know more of the inner workings of the stuff, I am very open to use this way to learn)