Hello,
I believe Matrix would be the most suitable candidate for your use case. The protocol supports both public and private (invite-only) rooms. It also has spaces, which are collections of rooms that helps with organisation (and yes they do exist in the sidebar). Voice/video calls can be done through Element Call which is integrated in many clients, and are usually quite performant. Pinned messages and polls are natively supported, and there exist various bots for reminders and other little neat features (see the Maubot plugins).
Matrix is also federatable like email, so you can extend your community to people on other servers in the network, too. Be sure to employ moderation tooling though, of which the ecosystem has plenty of and are improving every day. I also recommend testing out non-Element clients (such as Sable, Cinny, or SchildiNext) to see which one fits best with your organisation.
In another comment, you have mentioned the limit of 100 users. I believe this only applies for the Element Server Suite freemium solution, and so I ask, why not use another open source solution? My suggestion would be Continuwuity, a homeserver written in Rust with a very active community behind it. Continuwuity is generally considered much more lightweight than alternatives, and have been seen supporting sub-500 users just fine on a machine with 8 gigs of memory.
There's some other QoL features of Continuwuity you may be interested in, like auto-joining to a room after account creation, or registration via admin-issued tokens. It can also integrate with your favorite single-sign-on solution via OIDC as well. So yeah, feel free to ask more about it here, or take the next steps in the support room!
There exists other solutions as well, but I think they are not the best candidates for few reasons. XMPP (i.e. the protocol behind Snikket) is more lightweight, but its clients still generally lack support for group calls, pinned messages, and polls. Fluxer may have a better UI, but it is not federated from the ground up which can lead to problems. I don't think Nextcloud Talk offers federation either(?), but I believe Nextcloud to be a quite heavy, "bells and whistles included" software suite in general which may be too much for your use case.
I wrote this by myself using anectodal sources from the community and experience hosting the thing, and no, it never passed through any LLMs. Perhaps I should approach things with a less upbeat and more cynically curt tone.