Hi James -
I see no one has answered your question yet so I'll toss this idea out 'cause I hate to see anyone left hanging!
I've not done a tour of the type you're looking for, and my guess is that all tours which include the underground spend the same amount of time in it due to the limited amount of space; they have to keep moving the groups along. I could be wrong but I've never personally seen a tour that promises more time in just that part than other tours. If anyone offered such a thing, my first thought is that it might be Context, as they've been reviewed so positively for their educational tours by travelers wanting the more in-depth, scholarly experience.
But sadly, I'm not seeing anything on their site for a focus specifically on the underground. That said, I did run across a couple of 90-minute, zoom "Before you go" seminars hosted by some of their expert guides. You might sign on for one or two and then take your knowledge to the underground on, say, Coopculture's own tour (usually the most economical) or one by any number of quality tour companies, such as Walks of Italy, Roman Guy, etc? Here are the 2 seminars I'm looking at which may fit the bill:
https://www.contextlearning.com/collections/seminars/products/colosseum-underground-online-course
https://www.contextlearning.com/collections/seminars/products/rome-forum-colosseum-online-course
Additionally, they do offer a private Colosseum tour that might be what you're looking for but it's quite expensive, the underground is an added upgrade - the feeling I get is that the majority of the tour is above - and it seems to be available on a limited basis but you could enquire? I would ask how much time they spend on your subject of interest.
https://www.contexttravel.com/cities/rome/tours/colosseum-tour-with-arena-floor-priority-access