Your better option might be to go to a high torque mini starter. They don't seem to suffer from the heat soak problems.
I can't remember how you said your starter mounted to the engine. Does it bolt to the block with two staggered bolts or to the bell housing with three bolts? If you have the bolts in the block like the picture below then you should be using the starter with an aluminum nose cone if you have an automatic transmission.
The engine block mount. Shows the staggered and straight holes for the starter mount bolts.
The starter mount holes in the nose cone.
Here are the two style starters. The longer one is the high-torque and may be the one you need.
This shows the small starter to block bracket for starter stability.
I don't see any problem with your voltage drop readings in fact thay are very good and if this is the case with the original cables, then you didn't need new ones because the problem is in the starter windings. If you test the drop readings while the starter is hot then the drop may increase.
The reason I said you didn't need the 2 gauge negative cable, is because it is rated to carry more than enough current than your starter will draw, and also because the engine block is part of the return back to negative, and it has virtually no resistance plus the negative cable will be shorter than the positive ,which reduces its resistance as well. Makes sense to me.