I’m uncut and I don’t have a ‘moist, slippery glans’. I had a very sensitive glans; stretching reduced hyper-sensitivity, but sensitivity for pleasure hasn’t lowered at all, maybe increased. First times, manual stretching seemed impossible to me, but it changed fast.
About the grip, I think using the glans as ‘anti-slippery’ is the correct way; tightening the grip (so using perpendicular directed force) can put too much pressure on the dorsal nerve; pulling out at the base on the glans (so using axial directed force to stretch), means that the pressure on the dorsal nerve is lower.
Maybe the shape of the given penis plays a role, however.