I have read that April and May are high season in Italy. Is it going to be massively touristy in Florence and Rome the last half of April?
Yep! The traditional shoulder seasons are just as crowded as summer, since Covid. I speak from experience from my recent Italy trip this past May, 2023. If you do the popular tourist highlights, you will find crowds and need to buy tickets to galleries and museums in advance.
Good luck !
Both April 25th and May 1st are national holidays, so it is relatively easy for Italians to take four days out of work and turn them in a full week holiday. So the last week of April is always crowded. Some years you can even add the Easter week.
Extremely crowded. Plus there are two major Italian holidays (April 25 and May 1) and when the Italians are off work they love to tour their own country.
While it will be crowded, you will feel it more so in Rome. Florence is enjoyable at any time imho. Rome is packed most any time.