If you haven't booked flights yet, is it possible to reverse and fly into Venice and out of Rome. There are two good reasons for this:
- Logistically, it's usually easier to arrive than to depart Venice. There's public transportation to FCO from central Rome from very early morning and takes less than an hour. And taxis take less time (outside of rush hour) and are not expensive. VCE is not nearly as easy, takes longer and a water taxi is exhorbitently expensive. It's more time efficient to get to the island from VCE than vice versa.
- You are arriving after a long-haul, probably red-eye flight, maybe with a layover. You'll probably be jetlagged and drowsy, unable to absorb much of anything on your first day, maybe your second as well. Rome is a big, bustling, noisy city with major tourist sights. Venice is the ideal place to start: small, quiet with no sights as major as the Colosseum or the Vatican, for instance. In Venice, you can wander the back canals, ride the vaporetto up and down the Grand Canal, relax and simply soak up the atmosphere while you "recuperate." IMO Venice is very romantic.
I have also used Milan as a gateway to the north, though not after a transatlantic flight. You could fly into Milan and start at nearby Lake Como for a couple nights which would also be a relaxing romantic start to your trip. If you do that, consider also stopping in Verona for a day (there's luggage storage at the train station) or an overnight on the way to Venice. In fact if you keep Como in your itinerary, it probably makes the most sense to fly into Milan, since any other route will take you out of your way to get there.
The most efficient route from Venice to Rome is via Florence and the CT in that order, to avoid backtracking. The CT can be really miserable because of way too many day trippers. Others on the forum have suggested better places to enjoy the Italian Riviera. Another issue is that in spring, some of the trails may be closed for repairs because of winter rain and storm damage. Also it is still possible to get poor weather. If it isn't sunny, the CT villages aren't picturesque and hiking will be difficult.
For Tuscany, because Florence is has the best connections to the other Tuscan towns, you may do better to stay there. You can day trip by train to Pisa/Lucca in a single day, to others like San Gimignano by bus. I'm sure you can find small group day tours for wineries/vineyards/tastings. If you stay in one of the other towns, that means extra time to change locations and to travel to your next destination. Unless you want to rent a car and stay at an agriturismo for 2-3 nights (which is not at all a bad idea). Then you could either drop the car in La Spezia on the way to the CT (or other Riviera town) or back in Florence if you skip the Riviera.