I'd use the S-04 for a bitter rather than S-05. Bitter in a British style, and gets a lot of it's character from the yeast. I wouldn't even consider using S-05 in a bitter, and I wouldn't consider using S-04 in an APA.
Last week, I brewed a strong bitter, (style 8C rather than 8B). It was AG rather than extract, and had a slightly higher OG. (1.050 vs 1.046). The specialty malts I used were effectively the same as yours i.e. 1/2 lb Crystal and 2 oz Chocolate Malt.
I think the extract and grains look fine.
I would use all light extract, rather than a mixture of light and amber, but I think this would make very little difference. If you have amber that you need to use up, then use it. If you have to buy the extract, I would just get the light.
I think the hops could do with a slight adjustment.
Your OG is towards the high side for the style, but your IBU's are at the low end of the range. I would replace the 30 minute addition of EKG with a 10 - 15 minute addition, and adjust the early hop additions for an IBU of 30 - 35.
All in all, I think that looks really good for a first shot at a recipe.
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