lithium VS Signal-Android

That's a good point. Threema using standard libsodium cryptoboxes (iirc) makes this easier to integrate with than these Axolotl-like protocols, though Threema still uses a custom protocol to communicate chat messages so you'll have to implement that. Also, Wire has a bot API so you don't need to pretend to be a real client to integrate in a chat. https://github.com/wireapp/lithium/wiki

Afaik Signal doesn't have that, there only seem to be third party implementations for bots.

Haven't used Signal FOSS, but as it happens Molly bas just implemented OSM support, perhaps when it lands you might have better luck with it. Anyway, is there no issue tracker for that fork? I tried looking in their repo and found it disabled

Try using the Signal FOSS version from https://www.twinhelix.com/apps/signal-foss/

Yeah I followed that whole story. Since then some forks have been created that do not care what Signal Foundation thinks about using their servers. Molly and Signal-FOSS for example, both compatible to regular Signal. Those two forks will not keep SMS support, Molly never had it, and Signal-FOSS will most likely drop it because they think it's too much work to keep it while staying compatible and up to date. I hope there is going to be another fork specifically dedicated to keeping SMS support. We'll see.

You can get a completely open source version without the Google stuff here: https://www.twinhelix.com/apps/signal-foss/

Signal FOSS removes proprietary google blobs https://www.twinhelix.com/apps/signal-foss/

Signal - Molly , Foss-Signal

Maybe they had outdated info from when the server software was not updated for a long time. Or, it's regarding the dependencies on closed-source Google Mobile Services and Firebase libraries, though those can be removed... and have been in this fork.