Feasible to use Firenze as base camp for single-day there/back trips to Lucca, Siena, Pisa?

Welcome!

Yes those are all easy day trips
Lucca and Pisa can be combined
Siena needs a full day

Monday is a good day to day trip as most museums in Florence are closed on Monday

Train to Lucca/Pisa
Bus to Siena

How many total days do you have in Florence?
When is this trip?

How many days do you have for all of these places? How many days will you stay put in Florence to visit and see it’s many treasures and museums including “David?”

Thank you much. We are considering an April 21 to May 6 trip with a few days in London probably on the tail end (our flight would be through London). So maybe 7-8 days in Florence?
Grazie mille,

Yes, you can do that, but it would be a long day.
Suggest staying one night in Siena or Lucca.

that's what I did one year

Stayed just north of Florence and day trips out to other cities in Tuscany: Lucca, Siena, Pisa.

If possible, I'd suggest scheduling around the Easter weeks. I learned the hard way. So if you, can avoid the week before and after Easter, which is April 20, 2025. Have a wonderful trip.

April 25 and May 1 are also Italian holidays

We are considering an April 21 to May 6

Sunday 20 April: Easter Sunday (major Catholic holiday)
Monday 21 April: Easter Monday, or Angel's Monday (major Catholic holiday when all Italians take family trips out of town)
Friday 25 April: Liberation Day (Italian national holiday)
Saturday 26 April: Weekend
Sunday 27 April: Weekend
Thursday 1 May: International Workers' Day/Labor Day (European wide holiday)
Saturday 3 May: Weekend
Sunday 4 May: Weekend

You are traveling to Italy possibly during the absolute busiest two weeks of the busiest year (Jubilee year) in this decade.
I do not recommend embarking on any day trips on any of the above holidays or weekend days. (buses are also on reduced schedule on those days).
Tuesday 27 to Thursday 24 are better travel days, as well as Monday 28 April to Wed April 30.
It will be better to stay put in Florence during the other days mentioned on top. On Mondays the Accademia (David) and the Uffizi are closed, but most other places will be open, even on Easter Monday. It will be full, however, as many Italians will descend into Florence from nearby cities and town to join all other tourists from all over the world.

Tourism in Italy, and where I live in France, explodes after Easter. If your dates in April are flexible, you may want to go in early April rather than late April. Knowing this, two years ago my husband and I were in Naples and the Amalfi coast two weeks before Easter, returning home just after Palm Sunday. We encountered little heavy tourism. However, another poster on this forum visited the same area two weeks after I did and found it very, very crowded. So, I would always aim for before Easter.
It's not only tourists from all over the world visiting, but also this is when European school groups make week-long school trips. And, as pointed out, this is a Jubilee year which will bring even more tourists to Rome and other areas of Italy.

Otherwise, yes that is a good base to do as you plan.

Thanks, everyone, for your super helpful comments. Have switched our plans to April 1 - 15, first half+ of trip in Florence, remainder in London.