I really miss something like thematic "groups" or "channels" in Mastodon, i.e., a mechanism for making sure that all the people subscribed get all the messages published in it.

That's the #1 feature that allows separate communities to grow, which makes places like Usenet or Reddit or ICQ to become a global conversation where anyone can participate, and where private conversations between restricted communities can happen independent of a central server.

The "timeline" model where everyone publishes their own stream, and anyone subscribe to individual persons, is good for fast propagating of ideas and news; but it doesn't create "clusters" of like-minded ideas that you can follow, and forces everybody to curate their own inputs. This makes it really difficult to jump in, as building a subscription list that provides interesting content and avoids noise has a lot of friction.