@Racer70 interporting is common for Oasis class ships in the Med, and it is possible because the ship does the same circuit every week. If it’s Monday it must be Palma!
No the ship won’t be half full for some of the time as those who board in Barcelona will stay on for the full 7 days to complete their cruise loop, and those who board in Rome will also be on for 7 days for their loop.
It means that if you board in Barcelona on your 4th evening you may see suitcases 🧳 in the hall way as Rome is day 5 so the Rome lot will be getting off and the new Rome people will embark.
It has also been mentioned that those who board in Rome have issues with the app and not being able to book things as everything is geared to Barcelona being the main embarking port.
Look at MSC cruise and they will inter port at every port on their Med loop!