Cannondale SuperSubFive (kg) Hill Climb Machine

could be an easy way to save gs.

Steel screws and aluminium washers are 10g per wheel on my low resolution scale, the CAD model says more like 8g.

You could take another 4g off by using aluminium screws, e.g. https://www.pro-bolt.com/aluminium-countersunk-6x15mm-bolts-countersunk-washers-set-x-4-7.html

The two ends of the axle which you cut the thread into must be the same continuous piece of metal, many hub designs use end caps and rely on the skewer to pass all the way through to hold everything together and that won't work with a screw in each end. Also, anything with a 5mm hex broached into the end of the axle for use with an Allen key as part of the hub servicing procedure will have too little material left to make safe M6 threads. The correct tap drill size for M6×1 is 5.0mm for cut threads, but you can get away with its being a bit over, and it will be because the hollow axle is bored to clear a 5mm skewer. I'm assuming 7075T6 axles since it's a weight weenie bike, which unfortunately don't lend themselves to roll taps which would be better otherwise, partly because forging threads is inherently a good idea but mostly because you could get a full thread form out of the 5.5mm bore.