I've recently been adventurous and started playing around with a couple of different sumi inks. One is supposed to have silver tones and the other, gold. All I see is black unless I spread the ink fine with a brush...which I think is how the ink shines best anyway. I still love sumi, but I have learned from the experience and it is related to your woes over thick thins. All three sumi inks give thick thins, even with a light touch. Moon Palace gives me the thinnest lines of the three I tried. So I searched, and learned some more. Sumi is made from soot, and soot particles can be quite large, which would impede our ability to achieve the fine thins we seek. I still love Moon Palace, but I now accept that its thins are limited. I second
@jeanwilson recommendation of McCaffrey's. It gives amazing thins!
But yes.....practice is also a big factor. The more you know the forms, the less you tend to clench your hand and weigh down your arm. Remember to breathe deeply regularly as you practice to stay relaxed. Lifting the hand also helps to 'reset' the mind, enabling it to focus better on the next stroke sequence