Element/Matrix, plain old IRC

Mattermost is a great alternative.

Is it text only? xmpp should be lightweight and smooth sailing. prosody configured to never expire messages would give you a forever archive on the server. There are good mobile clients, plus web/pwa and native desktop clients.

Matrix and Element work. It has apps for desktop IOS and Android. Can have multiple channels for different topics. Not quite slack level of workspace management, but it's an option.

We use matrix with Element and set up our own video server so that we can do video meetings etc. All encrypted and self hosted.

Really depends on what you're looking for. I'm a big fan of xmpp and irc but that might not fit the bill

@Chemslayer I run a self-hosted Mattermost instance. It works like charm.

At 30 people, buying a refurbished rackmount server is my recommendation.

They run anywhere from $250 to a couple thousand, but I'm running a media server and ebook/comic library server on an HPE DL360 Gen 9 with 64gb DDR4 RAM and dual 8-core Xeon processors, which I paid $450 for.

That will give you so many more options for your co-op.