Copper indeed would be better. Silver would be better yet. These materials, however, are heavier and less rigid than aluminum. So there is that. To aluminum's advantage, it can be laser cut at very high accuracy. Copper has to be water-jet cut, at lower accuracy and higher cost. Beryllium copper alloy would be stiffer, but it's a health hazard and no shop will cut it.
Copper, in time, will also corrode. And as RF travels mostly on the OUTSIDE of things, that would be a big problem in time.
Conductivity in a Mag-Loop antenna's capacitor IS a concern, however, despite the low currents involved. You want a very high-Q capacitor for best effect. And ANY resistance will drag down the Q. So copper, again, if it can be used.
One solution which I'm presently contemplating, is use of silicone transformer oil as the electrolytic (versus vacuum or air). This gives a K of 2.71 and an anti-arc insulation of > 350 V per mil. Trade off would be the (still very low) loss tangent of 0.0001 up in the HF bands. That by itself would lower Q by a tad. But counter to that, you could have copper or copper-plated plates. Their being immersed in silicone oil would prevent corrosion.
As for the (usually Russian surplus) vacuum variable capacitors, those are truly excellent ... with one proviso. You will be constantly tuning your Mag-Loop antenna (since Q is high) and that would mean VERY frequent re-adjustment on your surplus (already used) VVC. Look at any picture of those. There is a copper (or copper plate) bellows inside. Constant adjustment would mean fatigue. Copper has LOUSY fatigue resistance. So then while it might last a good while, don't go thinking that means forever. It will wear out, mechanically, from constant adjustment. And where, down the road, will you find an exact replacement? So then, maybe buy two?
One final thing. Do NOT simply press-fit your plates together the way I've seen done on YouTube and elsewhere. All those inter-plate contact connections. What must be the ESR accumulation from that? Braze them together for electrical unity. Use AlumiWeld or something like. It's only 25% the conductivity of Cu, but better than a press-fit, for certain. Or maybe use 112 Cu (hand cut from McMaster-Carr stock) for the spacers. That 112 Cu comes in telescoping sizes, so you could have Cu spacers on Cu tube (which runs through the plate-holes, plus inside of all a length of stainless steel threaded rod to hold things together until it's all brazed.
I don't have a final answer to all these questions as yet. I'm only working out my design now. A project for next year. I'll write an article when it's done. Can't say when that will be.