I found snikket to be pretty much perfect for the use case I wanted, which was something locally-hosted I could drop in a VM so I had a channel for talking to bots that didn't need API keys for third party services.

The only downsides I've experienced are:

1. Images and accompanying text get separated, which turn-based bots get confused by. I seem to recall there's a standard for this but it's not implemented.

2. XMPP support isn't common in claw-type agents so I've had to patch it in. Not a terrible experience overall.

I had a look at Matrix but bounced off Element, hard.