Any wiki software for documentation for starter, then a forum if you'd like to provide support or let users help each other. Instant messaging should only be used for collaboration, direct messaging to the maintainers and hand-holding.

It is fine to use Discord, I think, as long as you keep in mind that, like every chat service, are going to run out of investor money and be bought or go out of business after a couple years go by.

The only successful business model for chat apps is giving away investor money in the form of free services.

Why would Matrix be a non-starter?

I've recently switched from Discord to Matrix and its been a breath of fresh air.

Hard to get advice from people who actually think Matrix is usable.

The reality is that a LOT of FOSS advocates are completely disconnected from reality and see no issue with that. They'll insist "there's nothing wrong with the UX" to their graves, no matter how many people are pushed away by bad design. They can't see it because it doesn't bother them, so as far as they're concerned the issues don't exist.

Then they'll make comments loudly wondering why <proprietary software> is so much more successful than <half baked FOSS alternative>.

Matrix is not a non-starter unless you're weird.

XMPP is an option but way less popular.

Discourse is an option often successfully used.

Zulip doesn't look pretty, but it does a lot of things right. I don't get the appeal of Discord though, and find it a usability nightmare, so I'm probably not the right person to suggest alternatives to it.

All of them. :)

https://github.com/42wim/matterbridge

Zullip and Discourse. Both excellent alternatives that are way more open (interop with email for example).

Why is Matrix a non-starter?

I hate Matrix, Discord, and Slack. I may as well use Matrix.