I'd say you're pretty safe with that recipe. Since you don't have any roasted barley or black malt, 500g chocolate seems fine.
I wouldn't bother with putting the chocolate malt at the end, I've done it both ways and did not see any difference beyond the process being messier and more complicated. It also depends on your water chemistry though, so might want to look into that. You generally need harder water for darker beers. John Palmer's book used to have a very nice chart for hardness of water vs darkness of beer, but it seems to have disappeared from the first few search results (along with the book itself, used to be available for free as a website). I'll have to check if I have it saved anywhere.
It also depends on your personal preferences, myself I like a nice chewy stout with enough roast in it to balance it out. For drier beers, I'd go for less roast.