Where to go solo after 8 days in Sorrento with family?

Florence first, it's the closest. Take the train from Naples. Then it depends on where you fly home from - Venice or Milan. The 3 cities roughly form a triangle. Do you really have time for 3 places? It will take you 1/2 day to get to Florence. and your 15th day will probably be your morning departure day.

Since CT and AC are so far apart, they do not make a good combination on the same trip. The biggest factor in routing is your flight home. Where is it from? Have your family made proper plans for their last night, since the AC is remote from Rome?

8 days is a lot for the AC, since places like Puglia are annoying to get to, especially with a big group. It's nice that you've been to Venice before, but there's a lot of Italy to see. To me, Florence is "art" not "Tuscany". Do you want to see Tuscany, which is good for a week, especially if you have a car. Don't overlook that AC hotels, Venice hotels, and The Last Supper all book up well, months in advance.

I don't think 8 days is a lot for the Amalfi Coast. I've done three trips there, two of them for about 8 days each and never got bored. If you base in Sorrento you have day trip options to Positano/Amalfi/Ravello, Capri, Ischia, Naples, Pompeii/Herculaneum, Salerno/Paestum. So I wouldn't worry about too much time there.

For the rest of your trip I agree Florence would be a good first stop, then I would do Venice and end in Milan since that's where you'll most likely find the best flight options (flights to the states from Venice tend to leave very early). I would just do those three as 'bases' but there are numerous day trip options from each of them. From Venice there is Padua (half hour, very easy) as well as Vicenza and Verona. From Florence the 'best' day trips are Pisa/Lucca (combined on same day, by train) and Siena (by bus). You can also do organized tours to smaller towns in Tuscany but those are harder to do by public transportation. But Siena is actually a Tuscan hilltown, albeit a big one, but if you take a walk around the 'edges' of the old town there are gorgeous views, and the town itself is gorgeous. From Milano I've day numerous day trips - Lake Como, Lake Lugano, Pavia, Bergamo, etc.

I've done lots of those places as a solo female traveler and don't do anything different than when I'm traveling with my husband or a friend. I perhaps make sure my hotel is good and central and has a 24 hour desk, but other than that nothing different.

Here's my photos of all those places - https://andiamo.zenfolio.com/f739967755
On that same site (under the 'blog' tab) is a trip report I wrote about one of my trips to Venice, Florence and Milano where I mention the names of hotels, etc.

I should have been more clear (rookie mistake :) about my 8 day tour with my family. We're doing a tour that our college is hosting for us. We stay in a lovely hotel in Sorrento (home base for the 8 days) and everyday we do and go somewhere different. Example: one day in Capri, one day in Pompeii, a day in Amalfi, etc... My family flies out on the morning of May 3oth from Naples, so I will go to Naples with them and here is what I'm thinking.......
May 30th am: Naples to Venice by train...I know it's about 7 hours, but figured I'd grab a bottle of wine and a sammy.
May 30th-June 1st in Venice (have an awesome airbnb)
June 1st am: Venice to Milan by train.....do I stay a night in Milan?
June 1st or 2nd: Milan to Cinque Terre by train: 2-3 days in Cinque Terre
June 4-6th: Cinque Terre to Florence by train with a quick stop in Pisa
June 6th or 7th....fly out of Florence to Seattle

My question: Does this make sense? Do I skip Cinque? I need to book my flight in two days so I just need to figure out where I want to fly home from.....

Don't leave your iphone lying around on the train asleep after drinking all that wine. Last May when I was in Italy a tourist did that and her Iphone with all her social media life, e-ticket stuff, etc. was gone when she got off crying "did anyone see a phone?"

I fell in love w/ Praiano. It was sorta busy but not too bad and filled with euro tourists (over American tourists). Positano was fun too but way more crowded than Praiano.
As far as where to go after that, I would suggest ending your stay close to whichever airport you are flying out of. It might also be worth it to just stay south and visit places along the coast (Salerno...Furore...maybe Naples). I regret not being more strategic in my first visit (planning visit #2 way more strategically).

Back at ya from Seattle!