What is the fastest way to be proficient?
I think the fastest way is race gate practice. Whether it's simulator or in real flight. Race training are the hardest to be really really good. And what it does is it trains on the pilot's reflexes, timing, stick movement and everything you would required in other FPV skillset. Of course it doesn't mean you automatically can just do freestyle. But one with race gate training would find transition into freestyle much faster and easier.