Threema
central, non free,	costs

positive: open source app with documentation
positive: functioning and transparent business model
positive: no cryptocurrency or online payment systems planned!
positive: multi-device capable
positive: independent of phone number as identifier
positive: customized version “Threema Libre“ for Android does not require proprietary software library from Google or other third parties
unclear: tracker in Android app “Threma” (as payment versions cannot be checked by Exodus). But no trackers detectable in Android app “Threma work” (36 permissions): Exodus (external)
positive: according to webbkoll (external) no third-party cookies and no third-party requests (third-party)
negative: central service (= dependency)
negative: no interoperability resp. no interface to chat standard XMPP
negative: server code is not open source
negative: no native desktop app

https://www.freie-messenger.de/en/warumnicht/#threema