Shortbread with a snap

Your recipe is a bit short of flour for a classic 3:2:1 flour:butter:sugar ratio, counting both the flour and cornstarch with the flour. You have 1298 grams of flour and cornstarch combined, it needs to be closer to 1386. A bit more flour will certainly help, but your method should also be considered. I would recommend cutting the butter in rather than mixing in order to deliver the best crunch.

Last, if you're cutting them into fingers then it sounds like it's pretty thick. I've found it's hard to get thick shortbread to be crunchy without overcooking it, I'd suggest thinner is better.

When baking use the touch test to detect when it's ready, you want to bake it until it firms up some, a few test batches will give you an idea when it's reached the point.

When making shortbread it is important to keep track of the temperature. Normally one is looking for 'short' pastry which is quite crumbly, and in order to achieve this it is important to have cold butter. If you want them to be harder, it may be as easy as to allow the butter to warm up a bit more.

Snap is unusual in standard shortbread. What gives cookies snap is a high level of sugar, with some butter. Think tuille cookies, they snap perfectly. You only have 35% sugar here, I would say increase that if you are willing to fiddle. Or start with a snappy recipe outright, for example a snickerdoodle recipe.