Quoted from Aurich:

I'd just use Discord. It has a superior interface compared to Slack (I would kill for Slack to have Discord replies instead of the awful thread feature). It's not locked down like Slack, very easy to have a public server if you want, or barring that invites are easier for private servers.
The new forum feature in Discord doesn't replace a true forum, but it is an easy way to silo specific reoccurring topics so they aren't drowned out by real time chat.
It's free, and the extra features (like larger file uploads) are on the user side, so the costs, if any, are covered by individuals in cheap month subscriptions and not borne by the people running the server.
I'm in Slack and Discord all day long every day, and Discord is so much better in almost all ways imo. Slack is for companies who need to pay for things like compliance. We have a paid Slack server and we don't have data retention at all because it's just fodder for discovery for lawsuits. So ironic.
I moderate a large Discord server (about 8,000 members) and run my own private server for my arcade, if you need any help I'm pretty familiar with setting things up.