I want it to be able to use it for continuous milling, like gear hobbing and milling springs, not just indexing!

I think it's best to have the worms axially stiff because they can't be backdriven. So if you just can keep both worms in contact with the gear teeth with a little rotational preload it should be backlash free and much stiffer than the version with axial preload of the worms or split wormgear!

Harmonic drive seems nice! But i like to design and build stuff more than paying for complete solutions

I was thinking of a gear belt drive at first.. but it's a little impractical when you want 1:100 ratio or so..