Advice planning train journey - stop in Pisa between Cinque Terre and Florence?

Basically there's no reason I have to book the whole itinerary in one trip right?

Right, but there is no reason to book the Regional trains at the counter, either. You can make separate purchases online months in advance, or five minutes before the trains departure.
Before getting on the train, Do not forget you must time-stamp Regional trains tickets locally purchased.

PS You should really learn how to correctly Spell the Italian city names if you want to find them on the departure boards. I have no idea where / what "Saranza" is.

You can go from Venezia to Manarola also via Milan and Monterosso

You should really learn how to correctly Spell the Italian city names if you want to find them on the departure boards. I have no idea where / what "Saranza" is.

It was supposed to be Sarzana. It is late and I'm tired. Typos happen.

You can go from Venezia to Manarola also via Milan and Monterosso

This is what I thought I was going to do initially but just about all the routes suggested by Trainline go through Firenze when I search Venezia to Manarola. I did manually search Venezia to Milano and then Milano to Manarola but that doesn't seem to save me any time.

Via Milan you don’t save time, but you might save train changes.

Spelling city names correctly is not trivial. Not too many years ago a Swedish couple made the news. They wanted to go to CAPRI, the famous island in southern Italy, in the gulf of Naples, but they misspelled the destination name in their GPS navigator and drove to CARPI instead, a large town in northern Italy near Modena. Once they reached the destination they went to the tourist office asking for information on how to book a boat tour to the Blue Grotto. They were really convinced to have reached the island of Capri in spite of the fact they never crossed any body of water on a ferry.

It's more about having reserved seats all the way to Monterosso and saving one change.

How do you know Trainline's site displays the full scheduled and all the fares?

Have you searched on the actual train company site?

https://www.trenitalia.com/content/tcom/en.html

Trainline is a reseller

It's more about having reserved seats all the way to Monterosso and saving one change.

How do you know Trainline's site displays the full scheduled and all
the fares?

You can reserve seats on regional trains?

I checked Trenitalia's website too. Same schedules. The minimum number of changes when I search Venezia to Manarola is 2.

The ones that go through Milano don't save changes and see to be longer journeys and get me there in the evening.

The journeys that go to Firenze then CT with the change in La Spieza make stops at Pisa. Can I just get off there and reboard another train later? Or do I need to buy a new ticket? I would think I would have to buy a new ticket.

You would need a new ticket

No you can’t reserve seats on regional trains

There are IC trains that come from Milan-those do have reserved seats

If you got to Pisa at 10 AM, you could probably do the main Plaza of Miracles in 3-4 hours. If you want to climb the Tower, maybe an extra hour. We spent 2 nights there, but we travel more slowly than some.