Tooling for hard milling has come a long way over the years.
People are machining 60 RC D-2 these days from the start.

Tool steel at RC 60 would be plenty strong. What type of cutter and
feed rates are needed to machine tool steel this hard ? I didn't think
anything would really cut tool steel that hard, let alone do it cost
effectively. EDM may be a way to go as well, but I'm not sure
expensive it is for two prototype pieces to test.

I think most tool steel at around 52-55 RC would have a yield around
300,000 PSI which would be plenty strong (280,000 PSI compressive
yield strength would be enough for me) . A2 or S7 would probably be
ideal, but I don't know if their available in pre-hard, or at these
hardness levels. Generally I had thought that anything at these
hardness levels pretty much had to be ground.

I've drilled and countersunk pre-hard 4140 (I think it was 27-32 RC).
I just used a new HSS bit and I was surprised how easy it cut but I
had always thought machining tool steel in the 55-60 RC range was out
of the question.

John