Maiori

As you plan to stay in Sorrento, I think doing Pompeii from there would be easier than from Salerno.

Perhaps add a day in Sorrento ( one less in Salerno?) and do Capri between the trips to Pompeii and Herculaneum. I would not want to do them back to back and certainly not in one day.

I believe Joe's right. I was looking at that part of Italy just last night, and the regular trains (serving Salerno) go to Pompei town rather than Pompeii Scavi. Rick says modern Pompei town is dull, and it's quite a walk to the excavations. Whereas from Sorrento you can take the Circumvesuviana (or the less frequent, more comfortable, more costly Campania Express) to the Pompeii Scavi station.

I have only done a one-day bus tour to the Amalfi Coast (and that was about 30 years ago), so I can't answer your question about Maiori.

I do, however, want to warn you about that night train you're proposing to take. I took that train in the opposite direction in 2015. It was the most miserable night I've ever spent on a train. In fact, even though I had a couchette berth and was lying down, it was really worse than an overnight coach-class flight to Europe. The train carriage jerked constantly from side to side (I'm guessing the rails are not in good shape), so I could never manage to fall asleep. I was utterly dead when I got to Catania, so I collapsed in bed and slept a very long time. Naturally, my body clock was all messed for more than 24 hours afterward. I encourage you to develop a deep interest in some place along the way, split the train trip over two days and sleep in a hotel bed somewhere. Anywhere.

Actually, why not fly? See what's available into Naples from either Palermo or Catania. Or--depending on what you plan to do after Puglia--you could check on flying from Sicily to Bari or Brindisi (connecting by plane two areas that are challenging to deal with by train). Then travel from Puglia over to the Naples/Amalfi area by train.

If I'm not mistaken, along the SITA bus route at Maiori, on the cliff side of the road there is a glorious little park, mosaic-like grounds. And in the afternoon sun in March, it looked idyllic.

Have no idea of the town itself, but we were perfectly fine basing in Sorrento instead of on the A.C., mostly because of the ease to get to Naples via the CV. Have also based in Salerno, which is a short ferry (in season) or (exciting) SITA bus ride to Amalfi or Positano.

Sorrento had some of the best cuisine in all of Italy, and Salerno wasn't far behind. In Maiori without transportation you might be a captive audience if it's more than 1-2 nights. Beautiful of course. And...Pasticceria Sal De Riso is in Maiori, probably the best bakery on the Amalfi Coast.

Although it is not a direct response to your question, I thought this might be useful:
https://community.ricksteves.com/travel-forum/italy/amalfi-coast-for-7-nights-itinerary-help

I have never been to Maiori. It depends entirely on your travel style and interests, but you need to reckon with how "isolated" (I didn't say, "remote") the AC is. It's actually good that Maiori is east of Amalfi, because the bus lines break there, and this will isolate you a little from high season, which INCLUDES the last week of May. (When we slept in Sorrento in the last week of May, the bus to Positano/Amalfi had more people in line every morning than would fit on the next, half-hourly SITA bus.)

As I said, I've never been there, but I did feel that I wouldn't want to sleep multiple nights in Amalfi or even (beautiful) Ravello. There just isn't enough to do, and it's too hard to take day trips. Maybe there are swell restaurants, or inland mozzarella farms you can arrange to visit, I don't know. But the taxi can be expensive. I guess what I'm trying to say is that Maiori isn't Bologna or Reggio Emilia "with water".

While you have accurate information about Pompeii Scavi, I have read of people taking a cab in modern Pompeii-city. You just need to know what you want to do in Sorrento. I presume the ferry is much shorter from Sorrento to Capri, and probably more frequent. (Multiple companies to Capri from Sorrento.)

It is not at all a must-do, but because you mentioned Slow-Travel, I note that there is Five-Site ticket including Pompeii and Herculaneum (limited total time span, I think) that includes three much smaller but almost equally important, art and archaeology-wise, sites. These are more easily reached by Circumvesuviana from Sorrento. Also note that there is a lot of art and history in Naples. Almost no one promoting the Naples Archaeology Museum mentions that they own the superb and famous Farnese Marbles. Although the title is off-putting, I thought the following thread about Naples was also, affectionate. It is true that Naples is crowded and frenetic, but it has a lot of back story, including That Hamilton Woman and the Borgias.

https://community.ricksteves.com/travel-forum/trip-reports/surviving-naples

Re: The ferries to Capri.

RS has a handy-dandy little transportation map for the area around the Amalfi coast in his guide to Italy. It indicates there's not much difference in travel time for ferries to Capri from Sorrento vs. Positano, but Sorrento boats are much, much more frequent and run year-round. And the dock situation at Positano (is there even a dock?) is such that the boats get canceled more often there due to iffy weather. Getting to Capri by boat from Amalfi or Salerno would obviously take a lot longer.