dreldas, if you want an idea of what will likely become available to you, search the Trenitalia site for the same day of the week as Sept. 22nd (Wed.) but in July. While there may be some changes between that schedule and when all trains are posted for the date you want in Sept, you'll see that there are currently no-stop "fast" trains running between Venezia S. Lucia and Firenze S.M. Novella stations nearly every hour. Journey takes 2 hr, 13 minutes.
Check Italotreno as well as they usually have competitive rates, although currently they are running fewer trains between those two stations. The trains for the 3 departure times they're running currently on Wed, July 14th have non-refundable tickets available for "smart" carriages for 39.90 €.
https://www.italotreno.it/en