Borghese to Vatican to Colosseum?

I would not count on public transportation getting you across town in 45 minutes, especially with a stop for gelato. I did this with a friend last May in reverse (tour of Vatican Museums/Saint Peter's starting at 7:30, tour ended at 2 - the guide was enthusiastic and went over the usual 5-hour time allocation). We took a taxi from Piazza San Pietro to the Galleria Borghese and got there just in time (2:30) for our 3pm appointment.

I think you are underestimating the toll visiting 3 of the major sites in Rome in the same day will take on you. I would redistribute your time.

To summarize your plan:

9am - arrive at and enter Galleria Borghese. Leave at 10:30.
10:30 - Gelato break and Spanish steps - 20 minute walk, gelato break, then taxi or metro to Vatican Museums
11:45 - (optimistic) - arrive at Vatican Museums entrance (make sure you have your ticket voucher in hand by then)
12:00 - (optimistic) - actually enter the Museums after security line
3:00 - line to enter Saint Peter's (security line)
3:15 - look around the Basilica
4:30 - climb the Dome of Saint Peter's (line for elevator to the roof, then staircase to the cupola)
5:30 - leave Saint Peter's
6:00 - Colosseum/Forum/Palatine (security lines again, no telling how long they will take but allow 15 minutes)
7:00 or sunset - Leave Colosseum/Forum/Palatine when they close.

There is no lunch break here anywhere, and everything must go perfectly (no wrong turns or waiting for buses or taxis).

The Spanish Steps is being restored, so you won't see the fountains operating. That would give you an extra 20 minutes.

Following day: transportation to the Catacombs/Appian Way can take the better part of a day, you may want to start out early and see the Pantheon (open until 7pm) and the National Museum (there are several branches, do you mean the one near Termini?) before they close. If you plan to walk along the Via Appia Antica, you will probably be tired by the time you get back on the bus to go to the National Museum/Pantheon.

You are really pushing it with times. You can cut the Borghese short at 90 minutes (they allow 2 hours) but go directly to thte Vatican Area and grab a bite at a bar near the museum. Grab a cab from the Borghese to the Vatican to save precious time. I would estimate €12.00 for a cab, well worth it. Skip the over-rated, nothing-to-see-but-stairs Spanish Steps for now, and consider going there in the evening IF you are not totally exhausted. A walk like Rick Steves Night Walk Across Rome is a great evening activity to pick up the sights that do not require more than a walk by: fountains, monuments, etc. You can even slip in the Pantheon in the early evening on your way between Piazza Navona and Trevi Fountain.

Vatican Museums, Dome and Basilica will take 4 hours easily. It's crowded and you cannot rush the crowd. So you're talking 12:00-16:00 most likely. Take a break and then an evening walk.

I think you should save the Colosseo/Forum/Palatino for the next day and go in the morning when it is not so hot. (Assuming you are here in summer?) Allow 3-4 hours for the ruins. You can then decide if you ahve the energy for the Appian Way or a Museum. :-) Maybe you'll just want to soak up some atmosphere, or take a rest and another evening walk. I think the museum is open pretty late, but I have not looked. A lot of museums are open until 19:00 or even 20:00, and are not as crowded during American dinner hour.

Zoe beat me to the send button but she lays our the day very realistically....

Thank you so much for your advice! We do want to try to see as much as we can, but also don't want to be too rushed and definitely don't want to be late for a reservation. It's hard to visualize how far away things are, how long lines will be, etc. We will be there in a couple weeks, so I'm guessing it won't be TOO hot yet.

Okay, a follow-up question: We will be in Rome on a Friday when the Vatican is open at night, so I had considered doing the Vatican at night to help extend our day. However, I didn't know how much we'd be missing with the closed galleries, and we also wanted to climb the Dome of St. Peter's so I wasn't sure how to schedule that in with the times. The earliest Vatican visiting time on Friday night is 19:30. The latest entry time for climbing the Dome is 18:00, and it looks like the crypt and altar area are closed after 16:00. So that's why we went back to thinking of doing the Vatican/St. Peter's earlier in the day.

Would it make sense to do St. Peter's after Borghese, perhaps fit in Colosseum etc, in the afternoon, and then come back to the Vatican at night? Or is that too much criss-crossing, and keep to your suggested schedule? If the Spanish Steps/Trevi are under construction, we can save that for a late night walk-by if we have energy, as you suggested. I know Appian Way/Catacombs will take much of the next day.

Good advice above but with one tiny mention: restoration of the fountain at the foot of the Spanish Step is out from under wraps:

https://www.skylinewebcams.com/en/webcam/italia/lazio/roma/piazza-di-spagna.html

But I think Trinità dei Monti at the top of the steps is still under scaffolding. Save this one for the evening after everything else is closed, or walk by it on your way to the Borghese early in the morning.

Whew- that's a lot for one day! I would strongly recommend visiting the Vatican first thing in the morning, perhaps on the other day of your visit. The crowds are enormous and if you go on a Wednesday or Saturday, take into account the mass performed by the Pope. During our visit there last week, our tour guide said that upwards of 100,000 people attend.Have a fun trip!

Thank you for your advice! We decided to make a Vatican reservation at 13:00 to allow more time between Borghese and Vatican/St Peter's for lunch and travel time. We will leave the Colosseum et al until the next morning, then Catacombs in the afternoon. We may have to skip the National Museum, Spanish Steps, etc, but after your advice we don't want to be rushing and overdo it so much that we can't enjoy it. Thank you!

Lunch in Rome doesn't start until 13:00 and is a lengthy affair. You will have to grab a sandwich at a bar (what we would call a cafe).

Seems more reasonable this way, may even leave a little time to smell the roses, or whatever. Do note that for your 9:00 am reservation at the Borghese Gallery, you are required to check in at 8:30 am or they can give away your reservations to others waiting in prey. Also, the Borghese Gallery is in the middle of the large Borghese Park, a good place to maybe smell those roses, take a walk, pedal a cart around, or even have a picnic before heading over to the Pope's place.

The Galleria Borghese has a coffee shop if you want to grab a quick bite. Do not eat the pizza at the Vatican Museums cafeteria.