The rental car shuttle is a nose bleed, but hopefully your arrival time helps. Even under good circumstances it adds 30+ minutes to arrival/departure. I believe the completion target for the SkyTrain is 2022.
Hopefully you're traveling through Arches National Park. I had to reroute a drive from SLC back east through there a few years ago and could only spend about 3 hours driving and walking at a half dozen stops. Some of the most beautiful scenery I have seen. Enjoy your holiday!