So in the first fret, my elbow is placed comfortably with a slanted angle, providing a good angle for my fingers. The thing is, the less slanted the arm angle is, the fingers change angle.

So during frets lets say 20.. my elbow wants to jab right through my stomach.
It's the opposite for me, my hand is more angled at the lower frets (likely because I play with the guitar on my left leg). You could always give that a go. I'm guessing this is less of a problem when you're standing (standing position is generally much closer to the position when you have the guitar on your left leg)?

I've been practicing with the guitar on my left leg for years now, never looked back. Problem is now I find it so much easier to play standing than sitting. Problems like this are much less apparent when you stand as you have more freedom with the position of the guitar.