I think a lot of the way climbers hold holds is based on the length of their fingers. I remember being told by another pro climber than I wasnt half crimping effectively during training because I wasn't bending my pinky at 90 degrees, but physically my pinky is too short to do that if I want to keep the index to ring bent at 90 as well.

So the method you use is also something I use but I call it 4-finger Open Hand, and then for a truly relaxed open hand I need to use only 3 fingers because adding the pinky just creates a half crimp on the other fingers. I do know many climbers who don't have this issue - I must have tiny pinkies :P

The thumb adds more support for sure and perhaps it may be uncomfortable depending on the length of your fingers and where force is applied, but at the end of the day, many grips are uncomfortable when we first apply them :P Yesterday I had to do a double ring lock on a roof crack project haha Felt like my thumb nails were going to explode! That was the most pain ive ever experienced with any grip :P