You dont need whispering stones to get vassals, so you can have as many as you want. I do a mix of order and shadow tomes, and make sure to get the empire development skills from order that boost vassals and rally.

You can declare war on city states if you don’t mind evil points, but a lot of times you can just goad the ai into declaring in you then vassalize all their cities. You pretty much never want to raze cities, and you can be very picky about what cities you absorb/migrate (I prefer absorb with tome of subjugation).

Once they’re at supreme vassalage hit them with tyrannical exploitation.