I wouldn't say that August is a busy time at WDW. Christmas, Spring Break, Thanksgiving...these are the busy times. Also, anytime after Galaxy's Edge opens will also be a busy time.
Buses are not very reliable IMHO. We have waited 5 minutes for a bus one day and 60 minutes the next. Their arrival is pretty unpredictable, even if the bus stop has the electronic signs that say when the bus is coming. Our 60 minute wait kept showing the bus showing up in 10 minutes, then 15, minutes, then 20, etc. It took forever for it to come.
The main reason for staying onsite is the ability to book FP+ 60 days in advance.