What do you think of my itinerary ?

It is a whirlwind but maybe you like to travel that way. Or it is your first trip

It looks like you have 8 nights in Italy? Correct? I think you have too many stops.
Take into consideration you will be jet lagged on arrival and by the time you get to Venice you will be tired and not want to do much. You really are only giving yourself one full day in Venice. It deserves more.
Also, every time you go to a different location, figure on losing half a day packing up, checking out, getting to and from train stations, finding next hotel and checking in. This is not relaxing.
Try to think in nights----
Nights 1,2,3 in Venice
Nights 4,5 in Florence (skip Pisa)
Nights 6,7 in Cinque Terre (or skip Cinque Terre and add to Florence) (or just stay one night in CT)
Night 8 in Milan

I agree that you need to scale back the number of destinations since you have so few days. If your top priority is Cinque Terre (note the spelling), then skip Venice or Florence to give yourselves at least two full days there. If Venice is the priority, then skip Cinque Terre -- you get the idea. If you are only taking a day trip to Pisa to get a selfie "holding up" the leaning tower, you could just fake it with Photoshop.

Keep in mind that most major museums in Florence are closed on Mondays. Since you only plan to spend a couple of hours in Pisa, consider stopping on your way to Cinque Terre rather than day tripping from Florence.

I was going to suggest what Frank has suggested regarding stopping in Pisa on your way to CT. There is a place to store luggage at the train station. Pisa is a stop worth a couple of hours at best. When are you taking this trip? That could definitely be a factor in determining if the Cinque Terre is worth going to.

we plan on sightseeing and relaxation

This is incompatible with your outline. As others have said, one less destination.

When do you plan on relaxing? You're barely going for a week...drop two destinations and enjoy the other two.

You will end up seeing the inside of a train or, train stations, more than you will see each destinations attractions. Your itinerary is do-able but, what's the point of traveling and spending your $$$ when you're barely going to be at any of the destinations?

Drop Pisa and Cinque Terre. On such a limited amount of time, it's not worth the trip to Pisa, and Cinque Terre could be cool, damp and still a lot of tourist amenities closed.

Take your time with Venice and Florence and enjoy some sightseeing and relaxation.

Way too much travel.

Why not fly directly into Venice?

Too many cities. You don't have enough time in Florence and really not enough for Venice.

This is not a relaxing trip, being on the train so long.

Consider spending 8 days in just Venice and Florence, with side trips to Siena and perhaps Pisa.