I think you have a bigger issue here: Radio inputs are unique, so the giant list is not needed at all, it only adds friction to the flow. You also say this:
So the hierarchy from top to bottom goes Community > Sub-community >
Thus, why don't you simply let the user select the Community, then display ONLY the available sub-communities for users to choose, then ONLY the available collections within that community?
These approaches can be achieved with a variety of methods, from simple page to page navigation to AJAX based selection, enable/disable accordions and so on. But the most important part is to guide the user to a narrow and filtered set of options.
This way, you reduce the friction in the flow while accurately guiding the user to perform the right action for the right content, minimizing the chance for errors