Just looking for opinions. What is the best way to get the thumb hole drilled. Insert, no insert etc.

Have you considered using a removable thumb insert? In my pro shop we use the brand called Grip-Loc. If you have any issues with yr thumb swelling or going down in size while bowling or have multiple balls that you use i highly recommend trying these.

i use the vise IT system which is a interchangeable thumb system...

it works the best for me because my thumb goes up about 2-3 bit sizes while i bowl..and all i have to do is have 3 different slugs and dont have to work out each ball and worry about 1 ball feeling bad and one feeling perfect...i end up having the same feel out of each ball with the interchangeable slugs

hope this helps