@Klimakipppunkt probably Threema Work.
#Matrix is used by the German #Bundeswehr, and #XMPP by the German #police btw.
Also Threema can see who writes who, while Signal technically cant, even if US based, sure there are other attackvectors, like via Cloudflare but one would have to trust Threema more than Signal.
Also #Threema doesnt have #PostQuantumEncryption as of now.
#SimpleX would potentially tick all the boxes, but it's still a bit small.
@desirable_dialogue