Is this itinerary too hectic?

Terri, are you traveling by train or car? I'm looking at transportation time between locations - some are quite long. Check each of those because some of your "two day" stops really would only be slightly longer than one day of time to enjoy that location.

Would you rather stay in ~Arezzo rather than Florence since you're planning to do day-trips to Assisi and Cortona during those two days? I'm asking because of time to travel and also the smaller towns are usually less expensive for hotel & eating, and you mentioned cost is a factor.

If your flight arrives in Milan early enough I would even consider going to Varenna first. A sleepy lake is better and more relaxing while you are trying to get rid of jet lag. You could then spend a night or two in Milan between Varenna and Venice.
3 nights in Venice is good. If you've seen it already maybe you have time for a quick visit to nearby Padua too.
If your goal is to have a base for Cortona and Assisi I would consider Arezzo or Perugia as a base, instead of Florence that you have seen already. Arezzo is also near Florence. No need for Viator. You can take a train yourself. But with only 2 nights, you actually have time for a day trip only. Remember that 2 nights is one full day, 3 nights is two days, and so on. Maybe you can stay 3 nights in Arezzo or Perugia.
3 nights in Sorrento/Amalfi Coast is kind of short. There is a lot to see there. But if that is all you can have...so be it.

It feels very rushed, to me. In order to meet your goal "to see the lesser-frequented (though still really popular) sites" and have time to enjoy those sites, I think you need at least three nights at most stops.

I think you need to skip something. Perhaps skip either the Cinque Terre OR Sorrento. The Cinque Terre deserve 3 nights since it's a bit out-of-the-way and since weather can really affect your enjoyment. Similarly, Sorrento is a bit of a haul south and I'd recommend 4 nights there, minimum, or you have to rush too much. So pick one or the other.

Personally, I'd keep Venice and stay north of Rome. Skip Sorrento and spread those nights over your other destinations. I am counting 16 nights in the following:

Keep your 2 nights Milan (assumes you have a list of desired sights and experiences for your one full day, otherwise move right on to Venice)

Venice - 3 nights

Varenna - 2 nights

Vernazza - 3 nights (too out-of-the-way to justify only 2 nights)

Train to Assisi or Cortona - stay in one or the other for 4 nights since you've not seen either but have been to Florence. Not much sense staying in Florence for 2 nights/1 day only to day trip to other places. I think every trip needs one "longer" stay, in this case the relatively longer stay being 4 nights in either Assisi or Cortona. Maybe take a day trip with someone guiding, maybe rent a car for a day or three, or just use bus and train to get around. So many nice spots near Assisi! (We'll be there in October for 5 nights, in fact.)

I can recommend an all day wine tour, if you are interested, with Gusto Wine Tours. I avoid Viator because they are consolidators/resellers. Best to contract with the actual guy providing the tour, IMO.

Back to Rome for 2 nights.

EDIT: Forgot to say: if you have more time than money, fewer changes of location saves a bit of cash, usually. Fewr train fares, for example, maybe rent an apartment and prepare a couple of meals.

Thanks so much, everyone. If our wish is to visit Assisi and Cortona, would it be best to stay in one or the other to experience the evenings there? I'm now thinking of spending 3 nights in Assisi, and taking day trips to Cortona and other nearby towns (suggestions?). Here is my modified itinerary (all by train):

Milan - 2 nights
Varenna - 2 nights
Vernazza - 3 nights
Assisi or Arezzo or Cortona (?) 3 nights
Sorrento/Amalfi Coast/Pompeii/museums at Naples - now I'm thinking 4 nights?
Rome - 2 nights

You'll note I've taken out Venice, because I'm quite sure the next trip will include Venice as we also want to do Dolomites and on to Switzerland then. All advice is welcome!!!

Keep in mind that a two-night stay means only one full day. (I have had plenty of those types of stays myself; I work for a living, and usually my vacations are about 10 days; you do what you can.)

One thing about traveling in October is the increased possibility of rain. Rain in cities is not as bad, because you can always do something inside. But rain in a location like the Cinque Terre would not be so great (and note the huge rains and flooding in 2016, not that I'm saying that will happen again). So consider what you would do, or how you could modify your plans, if it turned out to be rainy when you would be there.

You don't mention whether you have previously visited the hill towns in Tuscany and Umbria. If not, then I suggest you consider reducing the Cinque Terre stay to 2 nights and increasing the Arezzo/Cortona area stay to 4 nights. That way you can do a full day of at least 3 hill towns using either a guided tour or a rental car. There are a large number of interesting towns to explore, all within 1-1.5 hour drive from Arezzo/Cortona area. You can also easily visit Cortona and Orvieto using the train, but others are a bit more difficult to reach without a car. While the Cinque Terre is beautiful and fun to visit, I found it easy to explore, hike the trail between two villages, and take a boat cruise, all in about one and a half days. Even with travel to the area, I found 2 days there was enough for my interests.

Thanks for the replies. Thinking of adding a day to Tuscany/Umbria. Other than Florence, my experience of those areas has been limited to Siena and San Gemignano. Also thinking of basing ourselves in Perugia, being brave and renting a car. What do you think?

I like Assisi much more than Perugia but personal opinion. We stayed 3 nights in Assisi and made day trips from there.

You wrote you are going from Vernazza to Florence, staying there two nights and then train to Sorrento. So you have only one full day in Florence but you are planning two day trips. You are basically selling the same day twice.

Hi - no, the revised schedule is as follows:

Milan - 2
Varenna - 2
Vernazza - 3
Perugia OR Abruzzo OR Assisi OR Cortona - 4
Sorrento - 4
Rome - 2

Really confused as to best base for Tuscany/Umbria. Our main sightseeing wishes are Assisi and Cortona, and we could either rent a car, use public transit or take Viator-type tours. What do you think?

In the above post, I meant Arezzo NOT Abruzzo!

For me personally, Milan is a one nighter :)

The rest looks great!

Enjoy!

Thanks JJ. Haven't scheduled flights yet, and depending on what time plane arrives we MAY just spend one night in Milan.

Also, I've just about decided to make Florence our base for Tuscany, even though we've been there a couple of times. Whether we decide to visit other towns by train, rental car, or tour, Florence seems to have the best options.