But for ordinary folk tooting or tweeting or whatever else it’s called is just shouting into the void. There’s no discussion because nobody ever sees what others post.

That was a perfect description.

Microblogging is surprisingly good for discussions between scientists and other academics, but I think this feels like the exception that proves the rule — a particular scenario where it does make sense to follow people rather than topics

Apart from the "subscription list" system there are lots of other things that would not be hard to implement. For example if creators are unwilling to use hashtags, then just let users do the tagging. If you are scared of false tags, implement a system where multiple people have to assign the same tag before it becomes active.

Then you can search for or subscribe to tags which would make discovery a looot better.