October 2017 trip - balancing sightseeing with relaxation

I think Venice, Florence and Rome are a good bit to cover in ten days, and I would not add the Cinque Terre.

Will you have other opportunities to travel to Italy during the shoulder season? If not, and if online information suggests that you can be in the CT on a day without multiple large cruise ships in neighboring ports, you might want to consider a visit on this trip. I'm guessing it will be somewhat less a zoo in October than in the summer. I'm not sure about the risk of chilly, rainy weather, though.

As you say, constant dilemma especially true for trips to Italy since different sites/towns are so close in distance to one another.
Stay on your stated goal though of making it a vacation not a rush.

You have been before to at least Florence so likely have a better idea of how much time/nights you need in each place than we would and what sites you have seen and what you didn't.
I thinking seeing the Cinque Terre is a great idea but if trying to also see Rome, Florence and Venice in 10 days, no I don't think that is a good idea.
If you can set aside 2 or 3 nights to the Cinque Terre do it, otherwise save it for another trip.

Since you have already been to Florence I would shave off two days and go to Cinque Terre. You travel so far and to not take the short train ride to Cinque Terre would be a missed opportunity of you ask me. Looking at the train schedule it is a Two hour 15 min train ride costing 18.30€. I think it is totally worth it and all you need is two nights, first day you get in explore your town, the next explore the other towns by train. The scenery is totally different from Florence and Rome and so breathtaking. We still talk about the Pesto and Mussels! You will need to decide soon and book your hotel to get a good choice. We stayed in Monterosso in October and could not ask for better weather.

I also would simply stay in the Cinque Terre, instead of Florence. Or what other side trips had you meant to include from Florence? If you didn't see Siena before and were planning it this time, it's another good place to stay overnight for full effect. As much as you loved Florence, I felt that revisiting Florence and Rome too quickly (e.g., two years after my first visit) was not as exciting as new destinations.

Since you've been to Florence, base in Bologna this time. It's low key with great food and lots of day trip possibilities by train. Ravenna would be my first choice, then Ferrara, Padua, Parma could be in your plan.

If you don't have tickets yet, it's a better use of your time to fly into Venice. If you do fly to Rome, I think you'd be better off to take the train to Venice (or Florence/Bologna), start there and then work your way back to Rome . When comparing prices, take into account the cost of that extra train journey at full price. If you stay in Rome at the beginning you are also adding a hotel change to your short trip.

Fly into Venice, go to CT instead of Florence, then fly home from Rome. When I priced it, the difference in flying out of a different city was the same or less than the full fare price of a high speed train ticket to Venice.