Rome Itinerary advice for only a 1 1/2 day visit?

I would not spend time or money on a food tour. Research the places you may like to eat. And don't skip the gelato, at least once.
Visiting St. Peter's is a must do. Lines can be long, so go when they open. You will need to decide if you want to just visit the basilica or if you will take the time to climb to the top, visit the Sistine Chapel, or spend time in the museum.
Must do the Pantheon.
Must do Colosseum.
Must do Trevi Fountain, but not when it's crowded. If you go at 7-8 a.m. you can avoid the chaos.
I personally would choose a rooftop to enjoy the sunset. There are some that overlook the Pantheon.

Can you share how /where these nights fall in your itinerary?

If you are just flying in on that first day and will be jetlagged my advice would differ from a stay mid trip

Pick two, maximum three of the things that seem most interesting to you and focus on those. What matters isn’t what others think are the “must sees”, but what you think. Take a look at people’s suggestions and then decide your favorites.

For what it is worth, I personally would put the Vatican Museums and Sistine Chapel very low on the list. They were my biggest disappointment in Rome. Not because there isn’t amazing art, but simply because crowding made it very difficult, nearly impossible, to enjoy the art. I enjoyed my time at Borghese Gallery much more.

It is now possible to book entry to St. Peter's with an audio guide ahead of time at modest cost, thereby qualifying to use a much shorter security line than the general public is stuck in. That is a no-brainer. Check immediately to see how far out entries are available and watch carefully so you don't miss out. You really don't want to have to stand in the regular security line.

I agree that on a very short visit, the Vatican Museums wouldn't even make it onto my list. Going there takes a large amount of time, and conditions are unbelievably crowded. If you love art and want to see some in Rome, there are many other museums to choose from (and the Borghese is magnificent). There's great religious art in the churches as well, and you can usually walk right in, sometimes after paying a modest entry fee.

I've spent over a month in Rome over the years and have never bothered to go in the Colosseum; I'm just not interested in ancient ruins. A great number of people disagree with me, which is fine. I'm just saying you don't necessarily have to go inside the Colosseum. (That's another ticket you need to buy well ahead of time.) Seeing the outside is an impressive experience, one I would recommend.

Rick has several walking tours of Rome available in his Rick Steves Audio Europe app.