Signal would be a good replacement for private messages and groups. I'm in groups of hundreds of people there, I'm sure larger groups exist.

As to channels… seriously just set up a simple website with an RSS feed? That's the simplest. A lot of providers have free DDoS protection now as well. If you're worried about privacy and whatnot, choose a provider like 1984.is or FlokiNET.

The broader point is: we really need to get people out of centralized platforms and onto less gate-kept spaces. Because with centralized platforms it is always possible they enshittify or turn out to be bad in some important way, and when that happens, the network effects hold us and our audience ransom. Moving back to web is one way of doing that. Joining the Fediverse (hullo!) is another.

And yes, I am waiting for truly decentralized end-to-end encrypted internet messaging tools to become usable enough to replace Signal eventually. One thing I am looking at – and again, it is not ready yet! – is Cwtch. Another thing I am really hopeful for is the Veilid protocol. But these are still ways off from being ready for prime time and widespread non-techie use. One day though!