Glowing Bear has a preference to hide channels that don't have any activity.

As for using wee-slack in Weechat, go.py helps. I bind it to meta-g, it's a script that lets you jump to channels by typing the name. Also, /part any channels you're not interested in.

In Glowing Bear, meta-g jumps to the search bar that filters buffers as you type. Now that I think about it, a fuzzy search with ranking would be a nice improvement to reduce the number of characters needed.