I looked into the Riot frontend to Matrix, and I'm really not
liking their CSS choices. Too much wasted space. There is an option to fork the project and write a custom CSS theme, but I think I have a better idea.
I think Matrix could work pretty easily, in particular using
WeeChat, which is a console program that runs natively on OS X and Linux, but perhaps the main sticking point IMO would be the user experience on Windows, since I'm not sure it's very nice to make people install Cygwin just to chat. It might actually be just fine, but to make things smooth for people there are two other options: either bridge to IRC (which sort of defeats the entire purpose of switching away from it), or take a poll among Windows users here and ask them if they can tolerate
this (which seems to be the most mature (non-Electron) native Windows client available for Matrix).
Edit: on the plus side, there seem to be mature Android and iOS apps for Matrix, in case people care about that...