Slack’s free tier’s limitation without the ability to search for past chat is frankly awful. You lose history, accountability and archives and records to hold actions and people to.
Slack isn’t free to ‘not for profits’ in the UK unless you are a registered Charity. They work this out through a third party company, in America however, it’s effectively free game because ‘not for profits’ or ‘non profits’ are recognised alongside charities.
Discourse, and forums in general, can seem awkward, and may feel awkward, and that’s mainly because people are forced to link out and structure what they say, though it can be a little ‘slow seeming’ it allows for a reference place that can be searched.
Discord, as a chat medium, to me is better than slack, because it’s free, it has voice chat and it has some limited screen share capability, but it also has roles, moderation and it has searchable history.
IRC does exist, people use matrix, relays, and some bots to have things connected together, frankly there are xkcd comics that joke about ‘yet another technology’ and ‘how do we keep it all together’.
The simplest answer is stick to one solution.
We already have some telegram chat rooms, and they can become unwieldy when there’s a lot of people in them, and also its history management is awkward, and difficult to archive chats, in fact we ended up manually archiving some of the conversations on discourse for the sake of re-answering questions that were repeatedly being asked and annoying people.
So we have discourse. It’s on the site, I recommend people try utilising this first, and posting here, at the moment it’s mostly under-used, and appears to be able to fit the bill.