Nice diagram neptunes

I don't see the need to bend the finger that much. Lowering where the palm of the hand is rested on the left shoulder should give you more finger space and allow you to straighten it out. My left hand is quite a bit lower on the pad than my right, it also helps when needing to use the d-pad. The tip of my left index finger can't touch the middle-top of the controller the way I position the left hand.
It is still a bit of a problem and I don't have particularly large hands either.
I just did a test, and realized I use the tip of my finger. But I also tried using the middle of my index finger, and it felt a bit uncomfortable too...but it's hard to say without actually playing a game for a longer period of time.
Just put the GC 'troller in my hands and realised I contact with the base of last joint rather than in-between the two.