I’m late to the party here, but did you consider just paying for Mattermost? If it meets your needs, and your organization has 250 people, the cost for licensing is going to be relatively small compared to your IT budget (right?). They have “contact us” pricing, which means you can negotiate it.

Licensing representation matters

It doesn’t, because they’re the copyright owners. Think of their software as dual licensed: They run it themselves under a proprietary license, under which they reserve all rights. That has nothing to do with the AGPL version that they license to you. The AGPL doesn’t take away the rights they have as copyright owners, nor does it preclude dual licensing.

(Are you a bot? Your reply is written like ChatGPT, and it has that self-defeating logic that ChatGPT has sometimes… eg. you wrote that you disagree with me, but then parroted the exact thing that I said.)

This is flat out wrong. If you’re the copyright owner, you’re not licensing the code to yourself. The AGPL is the license under which they’re making the open source version available to YOU. The version they run themselves is proprietary.