Salerno or Sorrento for 6 nights

Deciding on Paestum would help. It is too far from Sorrento, but quite doable from Salerno. The temples are remarkably intact--more so than what I saw in Greece.
Salerno feels like a normal city, it just happens to be situated next to a gorgeous coastline. Sorrento has its fans, but it has more of a resort feel.
You can visit Pompeii from either. From Salerno you will get off at the Trenitalia station, but it's just an extra 15 minute walk.
The Capri ferry ride is longer from Salerno (check schedules at https://www.navlib.it/en/lines/salernomolo-manfredi-capri/).

Of all the areas you're considering in southern Italy, Sorrento is the best base. Salerno is good for getting to Paestum, but not very interesting or as beautiful as the Amalfi Coast or Sorrento. I did 4 nights in Sorrento, with day trips to Pompeii/Herculaneum, Amalfi Coast (Positano/Amalfi) and Capri. You could spend the other 2 nights in Naples, which would be an easy airport to get back to Paris, and you can also take the train directly from Naples down to Paestum one day if you want. Another easy trip from Naples is Caserta, which is only 30 minutes away and Italy's version of Versailles. I would suggest taking the train from Rome to Sorrento for 4 nights, then returning to Naples for the final two before flying to Paris.

Sounds like you need to cut out Salerno. If you’re not interested in Paestum Sorrento works best with your itinerary.

We really enjoyed our 2 nights in Salerno. We went there specifically for easy access to Paestum- which is absolutely worth the very easy train ride- fabulous!
But we had already spent 7 nights between Sorrento/Amalfi. Not sure I would base in Salerno but I would go back. It has a lovely waterfront with great views, a nice historic center, some great restaurants, excellent gelato and less expensive lodging plus direct access via fast train to Naples/Rome/wherever.
Not touristy at all.

You do need to decide whether Paestum is on your list. I encourage you to research it- the museum is really good there as well.
and there were no crowds at all- we were there in may 2019.

Then you could split 3 Salerno- visit Paestum, ferry to Amalfi/Positano/Ravello.
(We stayed in Amalfi and day tripped by bus to Positano, took ferry back to Amalfi- a few hours in Positano was enough for us. We overnighted in Ravello and it is lovely- the villa gardens were gorgeous and views are fantastic- but not good for a base and a bit too quiet/small.

Then spend 3 in Sorrento for Pompeii, Naples, Capri.

I just checked airbnb and Sorrento is considerably more expensive (solo traveler) than Salerno. I’m leaning towards Salerno for Amalfi coast, Paestum. The train runs directly from Salerno to Naples and Pompeii (I think I have extra walk that Sorrento route doesn’t). Maybe 2n in Naples for Pompeii and capri.

https://community.ricksteves.com/travel-forum/italy/amalfi-coast-for-5-days-where-to-set-base-from

https://community.ricksteves.com/travel-forum/italy/one-week-on-the-amalfi-coast

https://community.ricksteves.com/travel-forum/italy/amalfi-coast-for-7-nights-itinerary-help

It is questionable whether CT and AC belong on the same trip, but that is very much a matter of personal preference and travel style - as well as time available for the vacation, and at each stop of the vacation.

Do you understand that the AC books up six months in advance? (I don't use AirBnB myself, for moral reasons.)

I’m staying at cute little hostel RS recommend in SORRENTO for 6 nights next month. I know ravello is 30 min bus ride from amalfi town, can I do both of those plus Positiano in one day?

Only wrinkle in plan is Paestum, Decided on Naples for 2 nights for that.

By bus or by boat (except for Ravello)?

The information I have is that Sorrento-Positano and Positano-Amalfi both take about 50 minutes by bus. Ravello is about 30 minutes from Amalfi, so the total bus time would be about 2 hr 40 minutes, not including the time spent waiting for buses, which I would expect to be substantial. It has been reported that lines for buses can be long enough that not everyone fits on one bus, and that westbound buses sometimes bypass Positano completely because they leave Amalfi full. You'd need to figure on sometimes having to wait for the next bus to arrive.

The boats are somewhat faster than buses, but they are a lot more expensive. My notes advise me to show up at least 20 minutes early on busy days to avoid the risk of being bumped. The views would be different, so my plan has been to travel by bus in one direction and by boat in the other.

There's a commercial bus company making the trip between Sorrento and Amalfi, with a stop in Positano. You might want to consider using it if the public buses are slowing you down too much. The website is www.city-sightseeing.it. This solution is more expensive than sticking with the public buses. I believe you need to buy a day pass rather than individual segments.

I want to do the walk from Amalfi to Atrani, too, and there are sights I'd want to see (churches, museums and/or gardens) in each of the town, so I'd certainly not try to do all of this in one day. It clearly can be done if you just want to ride buses and boats and look at scenery. Beyond that, it depends on what sort of sightseer you are.

^^^ I saw that city seeing hop on hop off too but their website says due to covid they aren’t offering that route right now.

I think I will try and do amalfi and ravello first if I have to add positiano if not save it for another day.