It sounds like you are going to have a good trip. One thing to remember is Bryce Canyon is around the 8,000 to 9,000 foot level and could be cold in September.
It looks like Sunrise is around 7am in September. If you can get closer to Goblin Valley, either Torrey or Caineville or Hanksville, after your time at Bryce Canyon it would help to get there for Sunrise. It looks like its around a 4 hour drive from Bryce to Goblin Valley without any stops for photography. You can spend a lot of time at either place, but 3-4 hours in Little Wild Horse Canyon you can hike up towards the end where it opens up and back down with photography along the way. Do not get suckered into doing the Little Wild Horse/Bell Canyon loop. Bell Canyon is a little disappointing after Little Wild Horse and the lost time could have been spent in Goblin Valley.
I have only done a few slot canyons and was spoiled by my first, a 4 day 40 mile hike down Buckskin Gulch and Paria Canyon. But after that Little Wild Horse is an awesome little canyon that is easily accessed. The different patterns in the canyon walls along the hike are amazing.
Then there is Goblin Valley. I bypassed this state park for years before finally deciding to go. It is sensory overload, there are so many rock formations to photograph you just can not think. You can really spend hours in this fairly small park. For me Goblin Valley was very hard to get a good photo. I went in October and hit Goblin Valley for Sunrise, hiked Little Wild Horse/Bell Canyon loop then Goblin Valley again for sunset.
Sunset at Goblin Valley in late September should be about 7:15pm so it would be possible with a late arrival into Moab to be there for Sunset. It is a little under 2 hours from Goblin Valley to Moab.
If you have not been in a slot canyon before I would recommend Little Wild Horse Canyon first, then spend the remaining time you have at Goblin Valley. If you get there for Sunrise, hit Goblin Valley first, due to lighting, then the canyon.
Tony