CRAP!The most incredible tickling on my feet that I ever got was from Mistress Feral (long since retired) at the Passive Arts dungeon (long since closed) in about 2006. She strapped me down to a bondage table, oiled my bare soles, and used a horse grooming brush on them. The brush looked like this one:
https://www.statelinetack.com/item/cowboy-mud-brush-western-style/BJH58/
I struggled mightily against the straps and nearly went out of my mind with the tickling.
Have you tried a feather tied to a vibrator?