Sorry to jump in but NdAppy/KV: In Aus and NZ tom thumb bits are what are known as full cheek snaffles here in the US
Not a leverage bit at all just a difference in vernacular.
OP - The D-ring is a good bit if you want to get away from the full cheek effect of your tom thumb. If you are kind with your hands it will not hurt your horse in any way. If it works better than the eggbut, keep using it!
Importantly, when you have picked the best bit it is a good idea to stick with it so that you aren't confusing your horse too much with trying different mouthpieces.