Forget the crowded shuttle busses, eBike into the park
eBiking is such a great way to get around in many national parks that there are now rental shops in most places where it makes sense. It makes perfect sense in Zion with a protected road and path into Zion. A bike path leads from the town of Springdale (plenty of rental opportunities) to the Zion Canyon Scenic drive which is closed to private vehicles except a few who are staying at the lodge.
Bicyclists are allowed on the road, although when on a bike you must pull over for shuttle busses and in fact the busses are not allowed to pass unless you are pulled over to the side. Which is fine and easy to do.
eBiking in Zion is nice because you’ll probably want to take in some of the hikes such as Angels Landing, The Narrows, Emerald Pools, etc. and with eBikes you can save your energy for the hikes. In addition, if you really want to do Angels Landing without crowds you can get going before the first shuttle bus! Hard to get going for such an early start, but possible.