One of these flutes was still very sharp, so it should cut ok.
I did all the CNC work from one side, cutting small, 2mm deep, pilot holes, with a 3mm diameter slot drill, where the 5.5mm holes need to be. Clearing all the metal with the 6mm slot drill took 2 hours.
I cut the holes to 5.5mm afterwards on Dad's pillar drill.
For the counterbores, I countersunk them, on the pillar drill, oversize, then did the counterbore with a 10mm endmill.
There was only just enough room to cut the 2mm slot, but there was enough room, see video.