You may want to see if this depth of analysis is even needed. How many users will this DC support? How many secondary DCs will there be? Will they be single function or multi-function servers?
With the hardware muscle power newer servers are being shipped with, all this subdivision of AD functions becomes a moot point, (depending on your size).
Remember, Microsoft wants the multi-national company market. They're training techs to SuperSize all design plans in hopes that people start actually using their product in mega-corporations. That's their main reason for being so "nth degree" on their server recommendations for their products.
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let the wise listen and add to their learning,
and let the discerning get guidance- Proverbs 1:5
