Eventually I used some reasonable-quality (Bosch) cheap carbide tipped hammer action masonry drills, and reground them with a small diamond wheel, of the type sold for cutting new slots on woodscrews, in a Dremel.
Zero rake, and double ground/split point, but I held the bit on one hand and the Dremel in the other to do that, so nothing fancy (I'm also a bit blind at those sizes without magnification, and I did it largely by feel and angle
- and I ain't got many fingers, or much feeling neither. so it wan't that accurate).
The carbide seems much better than any of the fancy metal drills - this also seems to be in general true of lots of masonry drills - and patience seemed to be the watchword, though I abused them at least as much as I had abused the expensive Fhrhei drills which broke in (very) short order.
This was for a 4mm (when reamed) x 80 mm (eventually) deep hole in Inconel X-750 - the larger Frhihe drills worked a bit better than the smaller ones (but still not worth it for Inconel IMO, the carbide cracked and seemed to be of lower quality, I'd stick with masonry drills).