I stayed at this resort, and I'm sure one of the reviews you read was mine. I was pretty underwhelmed by the place, but I love the location. If you will only use it as a base camp, it's fine. If you want fancy digs, keep looking. To be fair, the unit I stayed in was the one closest to the office, and it was pretty run down. If they try to give you that one, ask for another. I'd stay there again, just not in that unit.
Weather in St. George itself should be pretty good about then. Zion will be similar temps, but may be a tad cooler, since it's a bit higher in the mountains. Bryce will likely be quite a bit colder, since it's a couple thousand feet higher elevation. But it should still be gorgeous to see.
Rather than drive all the way to Arches and Monument Valley from there, (which is a really long ways and should be a trip in itself), give thought to doing what we did: From St. George, see Zion NP, then drive up I-15 through Kolob Canyon, and on to Bryce NP. Take Utah Highway 12 to Kodachrome Basin State Park, then on to Capitol Reef National Park. Backtrack and cut south through Kanab, Utah, to the North Rim of the Grand Canyon. Come back through Zion back to St. George. It makes an awesome trip, with loads of incredible scenery.
When we did our week there, we stayed two nights in St. George while seeing Zion. Then one night at Bryce, one night in Torrey, Utah near Capitol Reef NP, then one night at the North Rim. Back to St. George for two more nights while seeing more of Zion. It's a fair amount of driving, but not too much on any one day. And the scenery is always changing.
Have fun!
Dave