I really don't have a problem with needing to be vigilent in guarding the basketball. I think this is exactly the kind of thing we take for granted with 2K. There's nothing wrong with asking the player to make sure the ball stays out of reach of the defender. As you just said, when you relax the ball gets poked away.
I do agree that it can be tiresome to have to dribble between your legs constantly, though. Maybe instead there can be a button that simply switches you into a guarded position with your body and non-dribbling hand out and between the ball and the defender. It can be the same button as post up but be context sensitive to the perimeter.
I know we can post up anywhere on the floor and thats a good thing but I don't know maybe while out on the perimeter a tap puts you into/out of a guarded position (not all moves should be available to you while you're focusing on guarding the ball afterall), but Holding it down while on the perimeter puts you into the post up.
I think it's correct as is to just tap the button when near the paint to post up, and that having to hold it that often could be annoying. But out on the perimeter where you will post up less often, having to hold it down wouldn't be so bad.