I have never shifted with just my thumb. Imagine if you extend your arm out a bit, and with your thumb and forefinger together in a circle like you're turning a dial, flex your wrist a bit.
I just love 'em. I have a bunch of Sun Tour Sprint shifters too, but have never tried using any of the mounts these dayze. The ones I have work fine and love that they are thick and stubby and ergonomic , which the Sprint/Silver shaped ones are not. I always thought they were too thin even as dt shifters. It befuddles me why no one but the "cheap" Sun Race and Falcon thumbshifters make them nice and thick with a rubber coating, instead everyone keeps copying the dt style and mounting them on some doo-dad. Jack of all mounts and master of none.
Shifting with thumb or forefinger. Very smooth, comfortable, no problems at all. 8 spd. cassette. Previous setup used Silver barends. Silver shifter now on the downtube for the front derailer.
Best of luck with your efforts!
Richard