I already know all this in-universe fluff, my rebuttal is that the Adeptus Astarte aren't don't really launch full scale planetary invasion and occupations, not by themselves. They normally have heavy Guard support, and usually have Mechanicus or Sororitas support depending of if they planet in question interest the Inquisition or Mechanicus.
So it doesn't really make sense to have an entire faction dedicated to just Space Marines. And like I said, you don't have to include every combat unit from every faction. Give us a decent selection of Guard troops, some Space Marine units for specialization, and maybe one or two units from the Inquisition and Mechanicus.