Even then, Mastodon and similars feel more like a market square with everyone trying to catch others' attention, even when they're all talking about a specific topic to "no one in particular". It's not as easy to follow a topic there as in a forum-style thread about the topic, like this one.

It's not nearly the same as following communities or groups, it's just a collection of posts grouped by tags, as opposed to a space where people discuss or post about a more broad topic. Also Communities and groups typically invite more interaction than simply tagging posts by virtue of being a place people post as opposed to simply being a post tag category.

I should note that there are groups on Mastodon (Not really in Mastodon itself but federated Group actors from other services show up there) though they are less intuitive and thus are usually overlooked by most Mastodon users.

There's a problem with that on smaller instances.

You can only see hashtags from people your instance already knows (someone follows them). On bigger, well-connected, instances this is not as problematic.

But, no matter the size of the instance, it just shows how even the "hashtag experience" depends on the "following experience".