First of all, I think you're sitting a little too high. Try lowering your bench to have your forearms about level with the top of the keys.
I don't think your problem is releasing arm weight but rather smothly passing the thumb without the up and down motion. The first thing I would do is to practice thumb passing exercises back and forth between 3 and 1 and between 4 and 1 (even between 5 and 1 once you get comfortable) VERY SLOWLY and focus on the smoothness of the movement. Think of it more as a lateral movement than an up and down one. There is a slight up movement but you have to minimize it if you want to play smooth scales at a fast tempo.
Also, minor point but to me it sounds like you're starting on an up beat and the scale is a jumble of notes instead of a clear line of 16ths. Having a strong rhythmic backbone will make your scales sound much smoother.