I count 5 travel days so I'd go with a 5 day Interrail Global Pass. On 2 of those pass days you can travel in your own country (including the Eurostar).

More for you to read :

https://www.seat61.com/how-to-use-an-interrail-pass.htm

https://www.seat61.com/interrail-and-eurail-reservations.htm

There are mandatory reservations on some trains along your route (extra costs). Eurostar needs to be booked well in advance while Italian high-speed trains have availability on the day.

You can travel on regional trains or at the front of the Bernina Express to avoid those fees. Excellent choice, that railway is beautiful all year round.

Pompei - Sorrento is a very short journey and that railway is not covered by Interrail. Tickets are cheap anyway.