I'm plenty familiar with Open Source and do contribute as well. But I would also be paying for Zulip if we were to move the company to it.

I think you missed my meaning.. Mattermost is open core and recently removed things from their community version. Also, it's really not cheaper given the features I need, so my concern is that I'm just jumping providers to another company that'll eventually pull the same rug. I like and want to contribute to Zulip to avoid that problem but am not sure if the product experience will work for my particular non-technical users.