We have used MM with now 450 users and Gitlab SSO cost free for years now at work. Can highly recommend it.

Rocket Chat was really never acceptable for us because of 2 main reasons:

• all users on the server are in „1“ team (there is no concept of „sub-servers“ or teams as in Mattermost.
• the cost free push notification limit was always waaaay too low for more than only a couple users.

And why would that be? They offer a generous free tier, because they earn their money with companies.

Tailscale as controller based Wireguard VPN is absolutely not comparable to this and way easier to setup and use.

But it can be, with https://github.com/juanfont/headscale .

Apart from that: it is a service with a 3 user, 100 device free tier which supports many other self hosted endeavors.