You can follow hashtags on the Fedi, and some accounts are "groups".

Feels like mailing lists.

I don't use that, but AFAIK on most existing social networks you can follow tags instead of users.

You can yes, but they're also primarily focused on following users. Can you disable that part?

I'm no ActivityPub expert but I'd want a software that does not publish individual user actions to activitystreams, only if they're done on a topic. And then you can only be notified of the action if you subscribe to the topic.

Kinda exactly like how reddit works but without the option to follow user accounts, that came much later.

The only thing I can think of is hosting your own ActivityPub server (e.g. Mastodon) and customizing the theme. Maybe that's the future of more individually-tailored online experiences.

I'm not sure customizing the theme will cut it. Because it's usually a background process queue that handles adding messages to an outbox and making them available for subscription by other users and instances.

So I think support needs to be integrated into the software.

Right, I partially misunderstood your idea. For notifications and such you would need to modify the source code at a deeper level. But for just customizing the home feed (update steam?) changing the theme might be enough.

Ideally there should be a personal ActivityPub instance implementation, something that doesn't require hosting all Mastodon content and can be easily customized in various ways.

Stacker.news tries to be a new reddit of sorts.