We did a mini tour of Italy by train in 2019, in mid August, and it worked quite well.
I had about 9 or 10 days off from work, and we got a deal where we flew to Rome, stayed in an air-conditioned hotel for 3 nights, took a lovely (? airconditioned) train late morning to Florence, stayed for 3 nights, took train to Venice, stayed for 3 nights, flew home.
It was originally a Groupon deal! which the travel company let us extend so we could spend an extra night in each place.
It worked out quite reasonably too. I think we were staying in hotels that were more business trade, and which were accommodating Groupon travellers when their business trade was quiet. Suited us, though. Very educational for the kids.
I booked us up in advance variously for an audience with the Pope, a Vatican museum tour including the Sistine chapel, the Academia, the Uffizi, Doge Palace tour and so on. These places were very pleasant temperatures, either air-conditioned or lofty buildings with lots of marble, or both. It was also mostly very comfortable temperature-wise on the vaporetti around Venice & the lagoon. Though a couple of times the vaporetti queues were a bit eyebrow raising.
I think that driving would have been a lot hotter and troublesome. Florence and Rome looked difficult to drive around and park in. I reckon Easter in Rome is also likely to be very busy.
I am reading with interest however, as we are seriously thinking about getting a van of some sort, and extended holidays in Europe would be great!