Report from 2-week Trip to Italy

Thank you to everyone on the forum who gave suggestions and encouragement when we were planning this trip!
We're now back, and happy to say we had a fantastic trip. Our dates were Sept 11-24. The weather for us was perfect. It was more crowded than we had expected. "Shoulder season" seems to be going later into the year now, as others have mentioned.
Our itinerary was:
Venice 3 days, stayed in Castello neighborhood; Florence 3 days (one of these days was a wine-tasting tour), stayed in a B&B a 20 min walk out of central town area; Siena 3 days, stayed in center of town, (see below); Rome 4 days, Trastevere neighborhood.
We used many of the suggestions from Rick Steves' Italy book, and found them to be helpful; this forum was fantastic for every little detail! For example, buying tickets in advance to certain museums: The Uffizi and Accademia in Florence; the Vatican Museums, Borghese Gallery and Colosseum/Forum/Palatine Hill in Rome. San Marco in Venice we maybe could have gotten by without an advance reservation. I waited too long to book on the official website, so I used a third-party (Get Your Guide) for that one.
We had a pretty aggressive sightseeing agenda, and saw a lot of famous places. Next time I might dial it back a bit. We are pretty active, but as the chief reservation-maker, I eventually felt that I was checking things off once something was "successfully completed". Thankfully, others on the forum had recommended cutting back my original idea to go 6 or 7 places; that would have been insanity. In fact someone may have used that word :) That was the best advice I got, and what I would pass on to other travelers. Enjoy the time you have, and the one(s) you're with!
Siena was for me a great break between a very busy time in Florence (made busier because we took a whole day for that wonderful wine tour!) and a very busy time in Rome. And speaking of Siena, I hadn't made any advance reservations so I felt less of that pressure to get somewhere at a certain time. And we still managed to have a great tour of the Duomo and museum. Also in Siena we stayed at Antica Residenza Cicogna, which was as lovely as the RS recommendation said it was. We had left a bag containing passport, laptop and kindle on the bus from Florence to Siena; host Elisa helped us figure out where to go and who to talk to; she also made a call to the bus station and gave us great guidance! We did not rent a car, and were very happy to use trains and buses. I had thought of taking a day trip to one of the surrounding charming villages, but it would have meant a long day of riding on a bus and transferring, so I appreciate the advice to stay in Siena and just enjoy the contrades/ambience there. (And we did see a lot of gorgeous countryside on our day of wine-touring in Chianti). Our favorite meal was in Siena at Bagoga, one of a list of recs from our host; (and in general we had great eating experiences, from gelato to fine dining).
We did a LOT of walking, which we both love. And it was harder than I expected! Cobblestones + hurrying to find out which line we were supposed to be in was often challenging. We had planned to take a train out to Ostia, as a substitute to going to Pompeii. However, we just ran out of steam and time, so those places will wait for a future trip. Also, the many bridges in Venice, though small, were frequent. We estimated we had 57 bridge crossings, according to my spouse, who loves keeping track of such things! So we were happy we didn't have more bags, especially upon arrival in Venice, when we did a lot of schlepping and transferring from bus to vaporetto to walking.
So, overall, a really great trip. Thanks again, everyone, for your advice!