First of all, like Janet I too start out with Rome2Rio to get basic logistics, then delve deeper on other sites from there.
I've been to the Amalfi Coast twice in early March, and weather-wise it's a crapshoot. One year it was sunny and 60, the next time it was 40's and blowing rain. That year our trip to Capri was cancelled. For that trip, we also based in Sorrento, which truly is the only town on the peninsula that is fully operational in March. Otherwise, base in Salerno. Both have great restaurants open all year, and I would give the culinary edge to Sorrento. Just fabulous food. Like stated upthread, it's a post-WWII town that's more touristy than cool, but when we were stuck there in the rain for 4 days it was plenty good for us.
If you are not renting a car, you are subject to public trans, which means the quasi-dependable SITA bus because the ferry won't be operating that time of year. From Sorrento we bussed across the peninsula to Positano, which was deserted (this was about March 10) except one restaurant. We had a great time view-wise with the whole town to ourselves, but there wasn't much open.
I would not do an AirBnB for any of those three A.C. towns for that reason, and the suspect transportation options. From Sorrento, it's a hub and you can also go the other way for daytrips to Naples or Pompeii.
Florence, Tuscany, CT & Venice after Orvieto? Wow--even with 2 weeks by itself that's way too much traveling I think. If possible you should fly in Naples, go to Sorrento for 2-3 days, train to Orvieto for only one night? It's beautiful, we made it a daytrip from Rome. Then train to Florence for 3-4 nights, then Venice the same amount and fly out of there. That's still a lot of moving around--you're going to be on trains, busses or planes a lot!
Any reason you're skipping Rome? It's my favorite city on the planet, just asking.
Finally, it's good you're addressing this now because with Jubilee 2025 country-wise, folks are definitely making reservations earlier. We're fully booked on a Florence--Rome--Salerno--Sicily trip for early next April.
Enjoy your planning!