I wish there was an option to purchase a film after you rent it. ...
Just because there isn't a
formalized option for this purpose, doesn't mean you have no options at all. Apple is typically very happy to refund a customer when they know that the refund is just going to immediately be re-purposed into a larger purchase. It's called "buyer's remorse," and nobody likes that feeling.
I purchased a Mac Mini from my local Apple Store several years ago, but realized shortly after unwrapping it at home that I would have been much happier if I had purchased the i7/1TB model instead of the i5/512GB model... I went back to the store several days later, and they happily swapped my purchase out for me with no questions asked. That was in 2012, and I'm still quite happy with that purchase; the Mini still drives my home theater to this day.)
In another example from 2016, I found myself chatting with an iTunes Store rep: my wife had purchased the first movie in the Harry Potter collection, fully planning to buy the entire collection... but for whatever reason, it simply didn't occur to her that she could have bought all eight movies as a set to reduce the overall cost. I came home from work just about as she was ready to buy the second movie, and I bought the full set for her instead. Then, knowing full well that Apple had no financial incentive to help me out in this situation, I went ahead and asked the question anyway... they cheerfully refunded me for that initial purchase -- again, with no questions asked.
So I would say, it's probably worth a few minutes of your time to at least ask the question. After all, what can it hurt, right?
