Hi photobear,
Ok, I understand about the mobility/being tired issues. But, if you change your plans, keep in mind as I suggested above, you can rest on pews at St. Peter's - unless you are not going to visit The Basilica.
Suggesting planning all three in one day - I figured it would be easy "since you are already in the neighborhood" of The Vatican, St. Peter's and Castel Sant'Angelo. But, you know yourself.
Also, keep in mind, you can rest a bit at little cafes or eateries in and around the Piazza of St. Peter's. Around the back of St. Peter's, is a neighborhood of little shops, little restaurants, cafes. And, no one will rush you away.
The Spanish Steps can be a quick visit. Those who can or choose to do so, climb the stairs to the Trinità dei Monti
https://en.wikipedia.org/wiki/Trinit%C3%A0_dei_Monti
Another point of interest at the bottom of the Spanish Steps, is one of the most famous squares in Rome (Italy). It owes its name to the Palazzo di Spagna, seat of the Embassy of Spain among the Holy See.
Nearby is the famed Column of the Immaculate Conception of the Blessed Virgin Mary. < That is where the Pope gives his Blessing on December 8, The Feast of The Immaculate Mary.
I don't know if you have considered this or mentioned it: Are you bringing canes to help with your mobility issues? Sure, you have to be extra careful on cobblestone, but, it most likely will help as a 'third leg' to take some of the affected pressure off. Just a thought...
Adding - some other fun facts about The Spanish Steps
https://www.thelocal.it/20160923/eight-things-you-should-know-about-romes-spanish-steps
The "ugly boat"
*The fountain at the foot of the steps is called 'Fontana della Barcaccia' or Fountain of the Ugly Boat. But why? A city legend says the fountain was built on papal orders, inspired by a flood which carried a fishing boat all the way to the square.
If it interests you: This is what takes place on the Feast - https://www.wantedinrome.com/news/feast-of-the-immaculate-conception-in-rome-2-2.html
It's crowded as you can imagine, but a sight to witness, for sure. Of course, it's only in December. But, I just wanted to give you ideas of the meaningful area in and around Piazza di Spagna.
Often, I like to suggest watching these https://www.youtube.com/user/TheRomanGuyTours/videos?disable_polymer=1 Browse the videos. You will get good views and ideas of what to expect, what's around, maybe what's off the beaten path, how to navigate around the city, and some fun tips.