I see why you want to avoid that, but why have the all-posts-tagged pages at all if that's the primary concern with tags? If you have all-posts-tagged pages, you have subreddits, full stop. #askhubski being a primary example. You can make them harder to get to, but that just means the people using them like subreddits will be the people willing to undergo temporary annoyance to use them like subreddits.

If you don't want tags used like subreddits, you need to do more with them than just allow following and add them to an all-posts-tagged page. That's a subreddit. You're just calling it "following" rather than "subscribing" and making it harder to get to the subreddit.

Why not replace "all posts tagged" entirely with some sort of discovery mechanism based on post context? Let's say I click #science, and I get a page populated with recent and all time popular (the latter should go away once I've seen them) #science posts, but also with posts that are tagged with keywords known to be related to #science, like #NASA or #biology, and with posts by the top posters who use the #science tag. That's actually functionally different from a subreddit and would begin to address the challenge of enabling discovery on Hubski. Just a rough off the cuff thought, but if you don't want subreddits, you can't just build pages that function exactly the same way and make them harder to get to.