Piano Forum

I think it helps to realize that:
a) Your pointer finger (2) is longer than your pinky, and
b) The thumb is a really strong and heavy finger, and moves downward mainly by the muscle (abductor and adductor) that moves the rest of your fingers sideways, which is slow and heavy (try it out with your other fingers).

For point a), you should try to have your hand in a position that gives 2 and 5 about equal height. Experiment with lowering your wrist just a tiny bit, from the forearm (which will also straighten your pointer finger a bit) and/or ever so slightly tilting your hand. Your pinky should naturally be further away from the piano (closer to you) than your pointer finger and it's easier to get sound there. Make sure you help your pinky with arm weight and rotation.

b) It helps me to feel like I should barely be "helping" my thumb with finger action, moving it mainly (sometimes even only) by forearm rotation (always to the left for the right hand). Make sure your wrist doesn't hinder your arm movement or your thumb will overcompensate, but don't over-relax it either (it will collapse and/or get very tired).

As a general note, playing too "fast" (not in tempo, but in how fast you move the keys) hinders control, so also dynamic control. Again from the forearm rotation, think of slowing down into and with the key. Especially the thumb tends to just fly ahead by itself. You can play very fast by "playing slow" like this!