That is a personal preference and only you can decide. If you decide to cut some of Siena to visit Venice, it would be difficult to also do the Cinque Terre. From Venice to the Cinque Terre it is a long train trip. You don't specify how many nights you have at your disposal. The ideal is always to spend at least two nights minimum at each hotel. When counting days, think of nights. Two nights at a location is one full day at that location. Travel days to transfer from one location to another are wasted days for the travel and the checking in and out process (including packing and unpacking).
Siena can be visited in a day, the hill towns another day, Florence needs at least two days, but just to get a taste of it, you can limit to a day. So in total you do need at a minimum 4 nights (3 full days) in Tuscany (Siena).
Venice alone needs a full day, so that would be two nights. Cinque Terre also needs a day from morning to evening, if you want to enjoy it and not just an in and out trip. Two nights would be good at the CT too.
If you have 8 nights available, you can do it all. If you only have seven, you can cut one night from Tuscany, but then you need to cut down either the hill towns or Florence.
Also for the locations you want to visit I don't see the need to rent a car, except for maybe a day to visit Volterra and San Gimignano or the Chianti.
All other locations you listed (Florence, Siena, Lucca, Pisa, Venice, Cinque Terre) don't need a car at all. You could base yourselves in Florence and do everything by public transport. Even San Gimignano.
