Kneeling on paving is not a comfort problem, it is a pressure problem. The pad spreads your weight over a larger area, and the ache goes away.
Thickness, honestly
4cm is the point where the pad stops helping you feel the ground and starts protecting the kneecap. Thinner pads bottom out, thicker ones make you unstable.
Water
Closed-cell foam does not absorb. Leave it out in the rain, wipe it, use it again.
Storage
It rolls and ties with the strap. It fits in a shed shelf gap.