Why not do this:
- use the router to spot the center of the holes
- manually make undersized holes
- finish them with a carbide 4mm endmill on the router (I would avise full depth cuts)
That way you won't have to mill hardened steel, won't put a lot of strain on the z axis drilling and if you take light passes with the end mill the machine won't suffer.
I'm sure that a 4mm end mill will snap off before putting too much strain to the machine
YES on the tool you showed
make shure you can insert CARBIDE bits
the standard may loose quick the edges on Plasma cut preholes
if you do a standard predrill they will work great
