Almost every lodge/camp in the Serengeti is all-inclusive in terms of food/drinks and game drives.

In terms of peak season, it depends on where you are (and when.) The Central Serengeti area is pretty awesome (almost) year-round, but the migration generally comes through during April/May and November/December. If by peak you mean July to October, then the Northern Serengeti (especially Serengeti Mara/Kogatende) is where the migration will usually be happening.

The vast majority of Central Serengeti gets jam packed with vehicles. The one exception is Namiri Plains Camp, which somehow manages to own a camp about 50km from anywhere else in a PRIME area. It's famous for having among the very best big cat sightings on the planet.

For almost $800 per person per night during high season, I think you can find much higher quality experiences than Four Seasons. It has a massive footprint (i.e. very heavily built), the area is congested with Land Cruisers and the people who stay there often care more about lounging by the pool than enjoying being one with nature. Safari parks are one of the exceptions when it comes to large luxury chains being superior to the little, independent guys. With very few exceptions (can't think of any, actually), they are not.