I am looking for an Open Source Chat Platform for around 1k non-tech users. I thought it is easier on the market...

Matrix: For non-tech end users still quite complicated when dealing with keys, encryption, etc

Rocketchat: Moved a lot of features to the enterprise version and the community edition is not really sufficient for such a use case

Mattermost: Moved some features to enterprise as well and the clients (especially Android) still encounter performance problems (e.g. startuptime of several seconds)

Do you know any good alternatives?