Ideally you would enforce these policies collectively by de-federating an instance
The developers do not control who to defederate from. Only instance admins can decide that. There is no collective decision to defederate; every instance must make that choice independently. It wouldn’t be very decentralized if there was such a collective.
You don’t need to be on a populated instance. You can just as easily access content from a small instance or even a personal instance. I’m not sure what you mean by the populated ones being closed off to you, but there are many instances out there, surely some of them align with your views. If not, you are of course free to start your own instance.