Hey Victors, I get what you're attempting to do here, but let's lay this out logically.
You've already written of the most easiest and simplest ways to proceed with the best results. You've said no aluminum, no 520 conversion, and not stock. So the difference in the steel options are likely only to save you a fraction of an ounce. And that small of a size would mean the rotational weight savings are negligible at best.
Given those limitations, find the smallest size sprocket to shorten the rotational weight and just drill a bunch of lightening holes in which ever one you can find the cheapest!
Oh, and stay far away from Supersprox.