DirectX 5, that brings back some memories lol. That’s when 3D started to get serious and it felt like there was a lot of innovation happening quickly (competition with 3dfx’s Glide?).

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.