Harder or stronger? With respect I think you mean the latter.

You might find stainless-steel studding suitable, but you would not be able to harden zinc-plated rod as it is just mild-steel.

One option is to see what high-tensile M6 bolts or socket-screws of appropriate lengths are available.

Otherwise you are looking at buying a higher-tensile steel (e.g. EN8) and making studs. If you do that, think on, so you give yourself a useful set of 6mm studs for other work-holding in future.

To be honest, if you need no larger than plain steel M6 studding then you won't be subjecting the assembly to milling forces excessive for such material anyway.

It might deflect though, so an approach I use sometimes is to add extra lateral support by suitable blocks, nuts and bolt in angle-plates, or similar, bolted to the table to the side of the work-piece and snugged against it.

If I need threaded rod to tighten up a bit more than usual I simply double up on the nuts