Just because somebody asked for an app to only post messages some days ago. Actually such an app exists for Hubzilla:
https://framagit.org/mario/hz-postit It supports textmessages, privacy groups and fileuploads. It is a proof of concept playground with react-native from some time ago... It does its job, but do not expect too much
If you do not want to build it yourself you can download an apk here:
app-release.apk
