I have worked with an NFP who worked with Mattermost and they were very responsive as backend support.
I have no exposure to pricing, but the fact they talk to people directly impressed me immensely.
IETF uses meetecho and it has meeting-support stuff including speaker control and voting mechanisms (I know, we dont vote in the IETF...) which I think are interesting. Thats more useful in the live online state. Again, the devs are unusually available.
I don't personally like discord, although many FOSS projects are on it. I think the whole stickers and like just .. turn me off.