Mattermost Team Edition
Team Edition is a free-to-use, open source, self-hosted collaboration platform offering all the core productivity benefits of competing SaaS solutions. It deploys as a single Linux binary with MySQL or PostgreSQL under an MIT license.
Are you thinking of using parts of Mattermost in your project?
This wouldn’t be part of a project, but rather just a tool I would use as-is.
The problem is that I can’t ascertain what features are part of the “core productivity benefits.”
The problem with the freemium model is that often you’ll need a specific feature that’s only available part of the paid software, and you end up paying more than you would with a closed-source SaaS solution.