John,
Looks good to me.
Most do drill the corners first, and it can be helpful.
Milling is a slow job, at the best of times. Especially in the home workshop.
A 6mm slot drill cutting 1 mm deep, now a 6mm slot drill cutting 5 mm deep - is removing 5 times more metal - thats a lot more chips to get rid of. The chips have to be removed as quick as they are being made. If the chips are not removed you will cut the same chip over and over again. There is just a limit on how much it can cut and if you force it - it will give up and break.

Have a quick look at this
http://madmodder.net/index.php?topic=4678.msg51465#msg51465may help a little.
Another way to cut (fly wheels) in the mill is to cut it out with a slot drill, then change to a 4 flute end mill, and finish it with small cuts to give a much better finish. This is the way I tend to do it.

With the end mill try to cut only in conventional milling manner.
With milling I find it better to creep up on it -so it don't see you coming

It is slow and steady.
Your fly wheel still looks good.

Hope this has helped a little.
DaveH