Actually, I rarely use nippers, but I do have a pair of the 14" 'cutting edge' nippers from Jeffers and have no problem with them. Maybe it's the tool or perhaps you are just underestimating the effort it takes. At least to me, that hoof wall is really a lot thicker than it seems when looking at the bottom of the foot that is not all cleaned up. There is no doubt in my mind, however, that professional nippers would be better/easier.
BTW, the angle grinder ($30-$40 from Home Depot) was a gift from my wife (along with a hoof stand) to save this old man's back Even still, I no longer do all four mares in one morning...being 6'4" means my back needs to bend a lot more than it wants to these days.
I think I am going to look into the angle grinder before I buy the high pricey nippers. Thanks, knowing that the same brand works for someone as well as what power tool to try helps quite a bit! Btw, I have the hoof jack, and it helps me too, and I am over a foot shorter than you! :) lordy! I think I have a small fortune in all of this stuff!! At least the angle grinder has some "other use" if I can't get proficient at using it on "practice wood pieces", or something. Meanwhile, the rasp works, its just more work. I am lazy.
She is in sand, so her feet are rarely packed. She has always been barefoot and does have thick walls for her size, though (13.2hh).