Almost all of the thumb holes I drill for people tourn into ovals because hardly anyone has a round thumb. The tape users do the same thing. They have a hole drilled a little large, usually round bu sometime oval and use tape to create the oval. The tape goes at the front pad of the thumb and the back of the thumb centered on the nail. Usually a rougher white tape is used in front and a smoother tape is used in back. I also like to round the edge of the thumb on the side where your callus is forming. I'll make sure I enlarge the hole a little on that side then use the triangle knife to round the top followed be the beveling sander. This helps my thumb come out smoothly.
I read that the IBPSA or whatever the letters of the Pro Shop
Operators are use a 40%-40%-20% rule foe thumb problems. Their training says that 40% of the problems are usually span related, 40% pitch related, and 20% hole size related. I don't know how true that is or if I got it exactly right, so this statment may be worthless to the conversation.
The more I think about it I think the person who said work the front/left of the hole(righty?) where your callus is to enlarge a little, round the edge some, and smooth it is the most practicle of the solutions. If that doesn't work it could be that there's a span or pitch problem but let's not go there yet. Your ball driller probably got that right if your hand doesn't feel stretched or the span too short.