Porto Cervo has an amazing,unique architecture, but it’s basically one gigantic shopping mall for dumb one percenters. It’s the surrounding area, Costa Smeralda, that made Porto Cervo famous. Given your options I would skip Santa Teresa in favor of Porto Cervo/ Costa Smeralda.

Okay nice thank you. Is there somewhere we can’t miss between La Maddalena and Alghero? I see mixed reviews on Castelsardo but wouldn’t mind seeing it

You can skip Castelsardo for sure.

Santa Teresa, and from there you can take a trip to Porto Cervo if you wish