The hole will need burnishing with a needle after sanding. A little -very- light twist is all it needs. Then an itty bit of very fine sanding, then another even lighter twist. For inspection try mounting the hole in your enlarger in place of a negative and examining the projection of the hole on the easel - it is the best way that I have found. Ideally the hole has a knife edge to it, but things are never ideal: to examine the smoothness and regularity of the side of the hole examine it with a loupe and glancing light that reflects off the sides.
The 'sunstar pattern' you are seeing isn't from the pinhole - the pattern is caused by the lens in your eye.
Pinholes work best at short focal lengths - 40%-50% of 'normal' is about right.
As to material, I've found 'coke-can aluminum' to be about the best: it is a good hard high grade of aluminum alloy. I roll it backwards around a tube to flatten out the curve. Next best is pie-pan aluminum - a bit thicker and flat, but it is softer so it is harder to get a really nice hole without tearing it.
The method (I am sure you know it, but repeated here for anyone reading the thread who is just starting out) is: Sand away the paint and lacquer. Then dimple the metal with the rounded tip of a 'knit fabric' needle. Do it against a hard(ish) material like chipboard. Then sand the dimple down - 600 or 1200 grit sandpaper flat on the table, move the metal over the sandpaper - to create the hole. That way you have sanded a (hopefully) knife-edged hole. The thickness of the material doesn't really matter all that much - if it is too thin it can't be sanded without tearing, it is too thick it takes forever to sand down. After sanding, burnish as above.