I almost always am wearing bike gloves, they help get grip on the shifter.
I am in USA. There is a product here called Shoe Goo that is similar to a glue, comes in a tube like tooth paste. You can put some of that on a shoe sole to build up the sole where it wears down.
I put three beads of Shoe Goo on my shifter, mine is the second generation of shifter, round in shape. I wanted to make it more triangular in shape. I applied two or three layers of Shoo Goo, it shrinks as it hardens.
Photos attached. Photos are nine years old, I am still using it the same way as shown in the photos.
I do not know if a similar product to Shoe Goo is made where you are or not.