Discord works well in practice for creating groups and channels. By default, regular users cannot create channels. Like Slack, it has very fast search, with no message limit. File and image upload work wonderfully and the audio/video works every time.

For small-scale groups, Group DMs are fine, though, I would warn to try to use Group DMs only for when more than two participants should get an urgent ping on a message. (Users can "mute" group DMs so they don't get notified by them.)