A change in my itinerary, suggestions?

We visited Rome last July and if you arrive early you should have no problem entering St. Peters (we arrived at around 8:30 and it took us around 20 minutes to get in although the line was so long we thought it will take us at least an hour). While we did not take a tour or climbed up the dome I would advise you to climb up the dome first as there was no line when we arrived, but when we were leaving it started to build up. We did Castel Sant'Angelo at around 4:30 PM after we got out of the Vatican museums and there was no line to get in so you should have no problem going there at 6:30 PM although I would go there a little earlier as in my opinion the castle (especially the view) is needs more time to be appropriately appreciated.

If by chance you'd like to schedule another tour, we really enjoyed going with Through Eternity. (RS recommends them too.)
I'm not usually a 'book a tour' person, but it was fantastic! The history, the ease of the entire morning... (we went really early).
Otherwise, you should be fine, whatever you decide! We went in September too and had an amazing time!

Thanks for the replies!

I don't think I want to add another 3rd party tour, if I did do a tour at the Basilica I think I'd just do a tour from there.

So, maybe climb the Dome first at 7:30 and then do an audio tour at the Basilica. Does that sound like a good plan?

Just so I'm clear--if I stand in line at the Dome climb after I'm finished with it I can go directly to the Basilica and get an audio guide without standing in another line?

The Trionfale Market is about a 5 minute walk from the Vatican and open on Sat from 830-1230 if you're up to heading out early yet being close to your tour time.