Hmmmm, I’ll just jump into two things here. Trying to break the record for hardest material cut with a MPCNC requires a much shorter machine, preferably shorter on all axis.
You will need constant cooling, a lot of it. If your bit gets red even one time it is garbage.
Manually feeding it will not work either.
I am pretty sure you can drill steel with an MPCNC but cutting steel is going to take a perfect set of circumstances.