Titanium Pickaxe

It breaks nothing that Mythril or Orichalcum could not already break, so the whole tier of pickaxe is optional. Bars are the bottleneck at this point and they are better spent on armor and a weapon. Keep mining with what you have and jump straight to a Pickaxe Axe or a Drax once the mechanical bosses are down.

If you do build one, take the pickaxe. It reaches further, which matters constantly when you are tunneling and want blocks a tile or two off from where you stand. More importantly it responds to mining speed bonuses from armor, accessories and potions, while the drill ignores all of them. The drill is only ahead on raw base speed, and that lead disappears the moment you put on anything that speeds up mining.