Curious why not something like Matrix/Riot?
Discord is not privacy-centric, at all.
Riot is another very good solution, akin to Mattermost. However, Modular (the private, hosted version) while cheaper than Slack is still quite pricey.
We are fine with Discord's terms of services, but I definitely endorse Riot for a more privacy-friendly solution!
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.