There is a train which takes 2 hr and 15 min and runs approximately once an hour. Are you looking on the trenitalia.it site? Put in any date in the next month or so to get the schedule. I can't remember when the summer schedule usually comes out but it is rarely significantly different from the current one so while you can't book the actual train yet you can certainly find out times and duration. Make sure you use Italian spelling for the city names.

Right. What "European sites" are you looking at?

This site has schedules for the whole of Europe: https://www.bahn.com/en/view/index.shtml and shows the trains Isabel notes. It is certainly far too early to look for trains in August.

You might benefit for reading the info on European train travel here: https://www.seat61.com/Europe-train-travel.htm

The two railway companies both run that route. You should be booking on one of their websites. August schedule won't be much different than the current one.

Right NOW the fastest timing from Santa Lucia to Santa Maria Novella is two hours and 13 minutes. The timetables will change again in June but no, you do not need four hours to get between the two cities.