What matters more to you - a maximally convenient itinerary or the freedom to choose, even if you have to spend time organizing everything yourself?
Kamchatka does have its quirks. There are roads between the main places (Petropavlovsk, the Valley of the Geysers, Paratunka), but not all of them are in perfect condition, and when it comes to specialized equipment for decent access to the volcanoes and geysers - that gets tricky. Planes fly there on a schedule that can depend on the weather. If you're going solo, you need to factor in car rental (foreign cars are expensive there), fuel, unexpected overnight stays if something breaks down on the road. Unfamiliar terrain, spotty cell service.
A tour saves you precisely because the tour operator has already dealt with all these pitfalls. The driver knows the roads, there's a set itinerary, meals and accommodation are already included, insurance against the unexpected. At first glance it looks pricier, but when you calculate the car, gas, shared accommodation for the whole time, and food separately - it often comes out comparable or even cheaper, not to mention the peace of mind. Plus September isn't peak season anymore, so prices might be more reasonable.