Congrats on your upcoming trip to Rome, jmormak! September will be a wonderful time to visit. My first trip was in October, and I loved that time of year in Rome.
I did the Roma Antica tour with Context Travel on that first trip to Rome. It covers the Colosseum, Palatine Hill, and Forum. Excellent tour, and the docent/guide Dimo had done his PhD dissertation on an excavation site within the Forum. Doesn't get much more authentic than that! To clarify, Context limits their tours to 6 people max.
Context also offers a Colosseum and Imperial Rome tour that appears to be focused solely on the Colosseum. Looks like a great way to immerse in that one site.
The only caveat with the Context tours is that these tours start first thing in the morning, not later in the day as you are looking for. With that said, I recommend visiting the Colosseum first thing in the morning since that complex (Colosseum/Palatine Hill/Forum) become a zoo as the day progresses.
If you're interested in a private guide for the Colosseum and/or other ancient sites, I highly recommend Agnes Crawford (www.understandingrome.com). I first toured with Agnes through Context (2016), then 2 months ago I did a Medieval Rome private tour with her. Oh wow, highly, highly recommended!!!
Let us know if you have additional questions, and happy planning!