Advice on my Itinerary

For my itinerary,
I do realize that the majority of you may think that I am spending too little time in Venice. However, in Venice, I do not plan to visit any museums at all but to just walk around the island

Have you considered flying into Milan and out of Paris to save backtracking?
Venice may deserve a little more time, as does Florence/Tuscany, although I can't fault the amount of time in Rome or Paris since they are so great. On a first visit, maybe balance your stops a bit more to better appreciate what is available. There is never enough time on any single trip.

Have a great time!

Yes I have considered that but I bought my tickets at a reduced price which only works such that the airport of arrival and departure has to be the same.

And yes haha I am also very optimistic about the Paris and Rome part of the trip since I’ve been wanting to visit those cities for several years already!

I think a few of questions are in order.....are your internal flights already booked? and what is the makeup of your 'family'?.....and are all hotel bookings made and non changeable?
Jetlag should be considered and for some it affects more than day 1. For that reason, I think you will still be suffering on day 2 and wouldn't recommend that day for a day trip to Siena. If museums are of any interest, I think Florence needs another day. The best you can expect to do on day 1 is walk around and get some natural daylight to deal with what most likely will be a sleep deprived flight over. That leaves you 1 day in Florence for any building sightseeing.
Keep in mind that rule of thumb is that for every move, you loose about 1/2 a day. Packing, checking out, travel to train/plane, checking in, unpacking......
I don't understand the addition of Venice at the end. For your limited time, it really isn't worth it. In my opinion, it would have been better to do right at the start - Venice, Florence. Rome, Paris. Alternatively, forego Venice, add a day to Florence and a day to Rome.
Please understand, as this is your first post here, you will receive a number of 'recommendations'. These are our opinions but ultimately, this is your trip. This is a better itinerary than some we see where people put in 7 cities and a bunch of day trips in 15 days.

Piu, have you already booked flights? If you have and they are non-changeable at this point, then suggestions for changes to most of your itinerary aren't really feasible. If you haven't booked your air, I personally would fly home from Paris instead of Venice>Malpensa and add that Venice day to Florence instead. It just feels like a lot of time/energy in transport for a very short time in Venice.

Editing: while I was typing you posted that your air is already booked so eliminating Venice/ flight out of Milan + adding more time to both Florence and Rome - which I would have done - doesn't appear to be an option. It'll work.

Thank You for your reply Maria. And yes, I have already booked the flights but not the hotels and train tickets. There will be 6 of us travelling including me, 3 teenagers, 1 adult and 2 elderly in their early 60s. As you suggested, to ‘counter’ the jet lag I will spend day 2 exploring the museums in Florence and day 3 in Siena instead.

Well for the part in Venice, let’s just say that it was the consequence of bad planning but I feel one day is enough (or not?) to see the surface of Venice though I wished that I could also spend another night there. What if I try to maximise the short time I have like leaving our hotel at 06:00 on day 15 to explore the Saint Marco Area and day 14 to just simply ‘get lost’ and take the vaperotto along the Grand Canal? Is that a good idea?

Thank you for your input Kathy, but sadly, I have already made my booking. I just have to maximise the short time in Venice and Florence I think.

...2 elderly in their early 60s.

Chuckling here as my husband and I are 64 and don't consider ourselves anywhere close to "elderly". :O)

If the two travelers in their 60's have any sort of mobility issues - which is what you mean by "elderly" - getting about Venice could be slow-to-challenging. You'll also want to be cautious to keep luggage as small/light as possible and book hotel rooms with lifts or rooms available for them on ground floor. Travel in Italy can involve dealing with lots of stairs...

More time in Florence.
More time in Venice.
Don't reduce time in Rome.

Alright then let’s not call then ‘elderly’ haha. Apologies if I offend you Kathy. But yeah they are in their early 60s but are certainly physically well and capable.

I understand your opinion Geo but sadly I do not believe that is possible.

I'm 61.....a lot of us are that 'vintage'. "Grandparents' could replace 'elderly' ;-)
Regardless, you can get a taste of Venice with what time you have. If you haven't booked your hotels, then consider something in Venice away from the train station and St. Mark's (but near a vaporetto stop). This will make wandering the streets more interesting and less about battling tourists. With your numbers, you might consider a water taxi from the airport. It might also make a gondola ride more cost effective, too.

Alright thanks. I will keep your advice on hand

I don't understand. It looks like you are pretty locked in, but ask for our opinion. At this point, I don't think our opinion much matters.

The one thing that might be changeable is the day trip from Florence to Siena. Drop Siena and stay in Florence. Your first day in Florence doesn't count.

I am concerned about your plan to be in Venice 6 hours before your flight out of Milan-Malpensa. I assume that's a non-EU flight, so your airline will probably suggest being at the airport 3 hours early. Trains from Venice to Milan-Malpensa take at least 3 hr. 32 min., and that doesn't get you to the airport check-in counter. I don't know how much extra time you'll need for that final bit of the journey.

Furthermore, the trip by train requires a transfer at Milano Centrale, with a scheduled transfer time of only 10 or 12 minutes.

Way too risky for me. Last-minute replacement airline tickets could be very costly if you miss your flight.