Don’t forget Porcupine Rim – classic. You might want to arrange a shuttle in town to cut out some initial road/track to get there.
In September it’ll still be pretty damn hot, so I’d recommend an early start and be off the trail before the full heat of the day. Take loads of water with you and don’t ride alone.
Amasa back works great as an evening ride.
Never had a problem with shops swapping brakes over, just remind them when you take it back.
I would contest that, perhaps apart from Slickrock, the rides on the whole are not that well-marked. +1 for the MTBR forums – it’ll be a whole lot more fun with some locals.