Nice job on the MQL mister, much less mess than flood coolant.
Couple thoughts on this. As you're finding out, carbide drills don't like opening up existing holes, they much prefer drilling from solid. All carbide drills are not created equal and there are tools designed for high hardness steels. OSG Tap & Die has tools that will drill into the 60's HRC, and I've seen demos of their tools drilling and tapping files. Many other companies have similar tools, but I'm not familiar with distribution in Australia. Word of warning, as the technology increases, so does the price.
Maybe consider trying carbide reamers. They are designed to open existing holes and can take fairly large depth of cuts. Might take a couple of different diameters to arrive at your final dimension. Or, if your finished dimension matches that of an end mill, feeding a four flute end mill with a large corner radius would cut that pin all day.
Not trying to complicate your life here, just throwing out some ideas that might work. Good luck!