0.020" ~= 0.5 mm. Yeah, that's about what I used. Maybe 0.55 mm.
Now, the holes. A couple of things to consider here. A big thing is the type of snow and the overnight temperatures expected. So much depends on that. But first ...
Some basic physics. Snow will bond to the surface of aluminium - actually to the very thin layer of aluminium oxide. It will bond very well in fact. Translation: you may need a sledge hammer to get the stakes out in the morning if the snow was at all near melting in the late afternoon. They may be rock solid!
It's all very well to say you can use an ice axe, but we don't carry ice axes when XC skiing. You can stomp on the top edge with a (ski) boot, but you risk crumpling the edge of the aluminium. That bond can be really strong.
So we move to titanium, for TWO reasons. The first is that Ti alloy is stronger than aluminium. It is denser too, but the greater strength means you can use a thinner gauge. The second reason for using Ti is that it does NOT bond to snow the same way. It will stick slightly, but not rock solid. Provided you insert the anchor at the correct angle (and you would be silly to not do this), then the Ti anchor will hold very well - just as well. But a slight tap can break the bond and the Ti stake can then be extracted fairly easily.
Now, what happens when you put those holes in? Well, if the snow is at -15 C, nothing. Big deal. But if the snow was slightly wet when you camped - and that happens a lot in Oz, then the snow can freeze overnight through the holes. That makes the stake extremely hard to get out, usually needing an ice axe or worse. The thing is, you just don't need that anchoring through the holes bit if you put the stake in correctly.
OK, cables through the holes. Beware the sharp edges on cut Ti sheet!!! Read When Things Go Wrong for some real-life graphic descriptions of what cut Ti sheet can do to Spectra! Cut right through!
Steel cable would be OK, or aluminium eyelets or pop rivets through the holes if you want to use synthetic string. I do the latter: it's much lighter. Then I use a loop of 200 (or 300) lb Spectra fishing line through the holes, with little Ti wire hooks. The fishing line is quite strong enough for extraction, and the loops weight very little.