Hello Emily, and welcome to the forum!
Your trip looks well planned and has generous timing in important places so great job overall. Usually people have schedules of 9 days and want to go 15 places that are at opposite ends of the country.
Visiting Orvieto will mean taking the slow train from Rome to the there and then onto Florence but since you're spending two nights I think the trade off is worth it. It's a lovely little town with an amazingly oversized church and will be welcome quiet place after Rome.
For a single day trip from Florence I would suggest Siena over Pisa as a more interesting town overall and more to see and do. I know some people love Pisa but for most there's not much beyond the Tower and the Field of Miracles buildings. Siena is reachable by both bus and train but many people prefer the bus because it drops you more in town. I think a spritz on del Campo - especially in a balcony seat if you can wrangle one - as the sun sets is one of the great low key Italian moments.
I would look to buy your tickets ahead of time for the things you really want to see in Florence - Accademia, Uffizi, whatever - because depending on what you mean by Spring you may end up wasting time in line for tickets. The Duomo museum is excellent but there are so many excellent museums and sights in Florence it will eat up as much time as you can throw at it.
Venice to Lake Garda is pretty easy by train but the train only services the lowest part of the lake. To reach further up on the lake you need to get off in Verona and take a bus from there. But this would probably start to make it too long and complicated for a day trip. Probably the train to Peschiera del Garda - an interesting little town but also has a lot tourist stuff built into it now - and take the ferry to another lake town to explore and see a circuit it the lower lake.
Naples is a lot. That's part of it's charm but it's a lot. Sorrento is a very upscale resorty town and is an option for Pompeii as well as visiting Naples as a daytrip as well. If you want to dip your toe in the chaos of the south you might look to Salerno. It offers access to Naples, Pompeii, Paestum by train as well as the Amalfi coast by ferry and bus. But the core of the city is walkable and super manageable compared to Naples with a wide pedestrian only shopping street corridor spilling into bars and restaurants towards the old town section. Salerno feels a lot like being a tourist in Italy in the 80s where you were never quite sure what you ordered and things were never quite how you expected but you just have to roll with it. I like that experience and little more of the southern chaos but not everyone does.
There many day trip and "you're close to" suggestions to be made but you've done a really good with a well paced schedule that I hesitate to add much to it.
Hope this helps, have an amazing trip,
=Tod