I am so happy to read this thread as I am going through some soul searching as well.
We are planning to move to a new (smaller) house so we have been purging a lot of things that we don't think will fit in the next home. But when it came to the pony collection - I couldn't do it - not even one or two - even though I know that space will be at a premium.
After talking to some other collectors, the best advice I found was to keep it - but instead of trying to display it all at once, rotate your display. They suggested to maximize the space you have, deep shelves from floor to ceiling will help you get the most out of limited space.
I'm hoping this is the solution. Stay tuned.