need help in planning my Italy trip

Your itinerary looks workable enough - I did almost that exact same itinerary the first time (Sorrento and Pomepii instead of Amalfi Coast though). If you go two weeks later then the crowds may be a little smaller, lodgings may or may not be a tad cheaper, but the days will be a little shorter, and you also start to run the risk of more rainy, colder weather. On my first trip to Italy I visited in early October and had great weather most of the time, but the last few days in Rome it turned chilly. The weather varies every year, of course. You are playing the odds of better weather if you go earlier.

I don't think it really matters a great deal if you go in late September or early October, then, except I can't speak about the Amalfi Coast. I'd check lodging prices and availability in Venice on both dates (on say Booking.com) and see if there is any real difference.

1. The only possible difference is in hotel rates. That is something you need to do yourself - look for places in each of your stops and compare prices. There will be more difference in rates depending on where you stay - especially on the Amalfi Coast, where staying in Salerno, for example, could easily be half the price of staying in Positano.

2. I would expect fewer tourists in October, but perhaps not so you'd notice in the cities. Maybe fewer day-trippers in the Cinque Terre if there are fewer cruise ships, and maybe on the Amalfi Coast.

3. It's hard to answer without knowing your travel style and what your interests are. Take into account that you'll use about 1/2 day changing locations, maybe a little more to the CT, depending on which town you choose for your stay, and almost surely a full day to go from the CT to the AC.

There will not be any significant cost between the 2 timelines .Entry fees do not change much seasonally . I would cut two nights from Almafi and add it to Florence and Rome or choose another city or Lake Garda for 2 nights . From Florence you can branch out on day trips to Sienna, San Gimingano ,Lucca ,Pisa... I am planning a similar trip. (9/25-10/9) Milan fly in, Venice(2) ,Florence(4-5) , Cinque Terre (2), Milan(3-4). Try to plan Almafi and Cinque Terre the first part of your trip. Hotels along AC tend to close for the season in October. The weather is still warm and so is the sea. I was in Positano late September and enjoyed the Festa del Pesce. This year it is 9/29 . It was a fun party with music ,wine ,fish on the beach and fire works. If you are going to the Vatican there, is always a crowd. Try not to backtrack on your trip. Try flying into one airport and leave from another. Book your flights first. Airfare increases constantly .

Looking at how your trip might unfold on a day to day basis, I think it’s an excellent plan. Perfect amount of time. Perfect pace.

1. arrive venice (3 n)
2. venice
3. venice
4. am..train via florence (store bags at station) to pisa - pm..return to florence (4 n)
5. florence
6. day to siena
7. florence
8. train to cinque terre (2 n)
9. walking cinque terre
10. train to amalfi coast/sorrento (4 n)
11. day to capri
12. day to pompeii
13. day to positano
14. train to rome (4 n)
15. rome
16. rome
17. rome
18. fly home