Ask HN: Having trouble picking chat back end, jabber vs. matrix vs. custom?

I've got a flutter mobile app to which I'm adding chat. I don't have any experience with chat protocols/systems/etc.

The chat requirements put it closer to Slack/Discord in that there will be a bunch of persistent messages in a bunch of channels. Users will be added to a few default channels, can find/join/leave rooms, and have 1:1 conversations.

I'm aware of XMPP, and ejabbered, so I am currently looking at what it can offer. I had not heard of Matrix, so I am checking out matrix documentation.

I would prefer sticking with open source off the shelf solutions that I can host myself, so building something custom would be a last resort. I also do not want to go with Twilio or similar SDKs.

Anyone have experience with adding chat to a mobile app?