We stay at OKW the first week of May almost every year... but we're missing it this year!
I think the bus service at OKW has improved in recent years. It appears that they've added buses at busy times. So in the morning, you'll see numerous buses to the parks. Keep in mind, some of them may be full - but you don't have to wait for that one bus to go empty and return. Another bus is usually right on that bus' heels.
Same goes for park closing - the buses are lined up to go to OKW. If you miss one, another one will be coming soon. There will be crowds, but I think they handle them very well.
That said, if you want to catch a bus somewhere in the middle of the day, you may have to wait a few minutes. There aren't any extra buses during off hours. I think the longest you could expect to wait is 20 minutes or so, if you happen to time it completely wrong.
If that will really bother you, I'd suggest renting a car. We usually rent one so we can do some off-property things. We drive to MGM, Animal Kingdom and the Marketplace because it's easy and faster than the bus. The bus is almost always faster (and easier) to Magic Kingdom. Epcot is easy both ways, but it's easier to get home via the bus than fighting the traffic with your car.
Have a great trip... I'm jealous!
Jon