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If I wanted to migrate from Discord, I'd give [Revolt](https://revolt.chat/) a try.  I don't know what the administrative load looks like, but it looks a *lot* like Discord, with servers and rooms, and down to the until-recent non-unique usernames with numbers appended.  That might have a better chance of people moving over than the usual gripes about "learning a new system."

That said, I never used any of them regularly and haven't recently, but [Mattermost](https://mattermost.com/) and [Rocket Chat](https://www.rocket.chat/) have been around for longer, look like the also-familiar Slack, and have fans.

I also haven't found any communities on it to test it out, but [Movim](https://movim.eu/) has built something Slack/Discord-like on top of XMPP, which should mean that the house of cards doesn't come tumbling down if they stop development for whatever reason.  Also, since you can connect with any XMPP client, that might should make it the easiest to "integrate" with the site, by embedding a web-based client like [Converse.js](https://conversejs.org/).

Likewise, IRC presumably going anywhere anytime soon, even though I haven't used it in a *long* time...

I'm a personal fan of the federated [Matrix](https://matrix.org/), but its structure is designed for a weird (at least to me) use case, where "spaces" look kind of like Discord servers, but are an arbitrary grouping of rooms and spaces that won't join you to anything in them.  I feel like that'd be right out, for this sort of community.

I realize that's more hand-waving and broad feelings than the question technically asked, but might help narrow down the priorities.  If comfort of the existing chat-users is the big issue, I'd definitely look at Revolt.  If embedding and stability are, then the older protocols with more implementations are a safer bet.