There’s also the more recent Alta and Solitude from Libre. They seem to be supported in recent CoreElecs nightlies.

But yeah, information on those is also a bit thin.

The funny thing is, that the Bundeswehr actually has a communications platform based on Matrix: bwMessenger goes live for Bundeswehr, element.io

Why they’re not using it? Who knows…

Maybe a LePotato ? No first hand experience, but it seems to be able to do 4k