I think we can’t help you far with that. Call your favourite PCB manufacturer and ask him for their HDI capabilities and rules. With via in pad you might also consider via plugging and overplating as well. For prototyping it might work without but you should have some x-ray on hand to verify if you have excess voids in that case.
You also have to think about whether you can layout with staggered vias or you have to pay for stacked vias. Keep in mind that the aspect ratio for microvias (independent of plugging) is 1:1. So you might rethink about the thickness of your outer dielectric.
Best regards, Martin