Never made brakes from scratch, but I was a machinist for a couple decades in a previous life.
Unless you buy ground stock, sheet metal or raw stock is not flat enough to use "as is", plus you want to "descale" the surface by removing at least .010" from each side if it's raw stock.
If it were me, I'd start with ground stock, then mill it on a rotary table, or CNC mill, or water-jet, or laser the pattern on it (any method will work), EDM is going to be the most expensive and you will want do it "wire feed". Make sure you use coolant during machining to reduce warpage. Then check for flatness on a surface plate. If it's not within .010" TIR (high low flatness), then either straighten it on a press, or better yet, surface grind it, or turn it on a lathe. Surface grinding can be tricky sometimes, depending on the thickness. If the disk is less than ~.187 thick, the surface will "pull down" when you turn on the mag chuck, and the flatness won't change. So you will need to check it with a surface gauge or height gauge, and shim it in areas that it pulls down. For LSR, thicker disks should be better.
Might be easier and cheaper to find an existing disk that is the correct dia, then make an adapter to mount it to your existing hub. Or get a caliper with a bigger swept area.
Can't help much with material alloy. Cast iron is the most stable common metal (won't change shape when it heats), but not sure what they use on newer bikes.