A workable flow
Begin from Arusha, spend the first day in Tarangire, move into the central Serengeti for two nights, then return through the Ngorongoro highlands for a crater descent before the final transfer.
What to protect
Protect one full Serengeti day and an early crater descent. If the road time feels too heavy, ask about flying out of the Serengeti rather than sacrificing the best wildlife hours.