I host Rocket.Chat for my wife’s business. The free self-hosted version now allows up to 50 users before having to purchase a license (was 25 prior to the version 7 update). If you kick a user you simply deactivate their account and regain the user slot. Rather than deleting a user completely which would additionally delete any messages they’ve posted.

Easy to host, easy to upgrade. Mobile apps both IOS and Android work well.

I wanted to use Zulip as I felt the interface was cleaner. But the 10 user limit was a deal breaker.

Good luck. Fingers crossed it’s your solution, and an easy one at that.

Quick web search led me to this which is an Acer owner trying to install PopOS. Likely similar steps that you will need to follow to be successful.