Did you look at/consider zulip? Using it at work now, and miss a simple/irc-like cli client (there's an official client that runs in the terminal - but it's more of a TUI than a weechat-work-a-like. However their python Api is nice enough that one can almost just use that. But maybe only for writing - not full participation in conversations... :)
Nope, didn't know about it. At first glance, it looks like the pypi package is for sending messages only. Since it's all open source, I guess it should not be too much work to get a slidge-zulip plugin working.
Feel free to open a ticket about it in slidge's todo.
Thanks for the information! I added this to my tracker then. Not sure I'll work on it soon, but at least the relevant links you gave me are stored somewhere easy to find now.