I think you misuse the term enshitification here. E. is intentional and top-down, what you discribe is a demographic shift that you dont nescessarily have to wall off. A reputation system, like here on HN or user customizable filters, can work in your case too. The difference is, is a top-down authority in charge or entites below.
I totally get it. Having small semi-private bubbles escapes the top-down. There will always be someone operating a forum but anyone can run a forum, especially today with tools to automate deploying them. I see each forum as a distributed directory of categories that are independent from the corporate capture top down. No corporate, no capture, only as censorious as each admin. Forum operators are only beholden to the law. Reputation systems are no perfect as they can amplify echo chambers which HN and Reddit are certainly not immune to. They are quite the double edged sword and create their own sub-class of enshittification which I must compensate for by removing karma from my view. Nothing is perfect nor ever will be. I will always use a balanced combination of walled off forums and bigger platforms like this to dip my toe into the pool of the masses which I can escape any time I want a break from it.
