Wow!!! Having had the opportunity to explore Sardinia myself, in a year when I bought an Interrail pass, I feel deeply connected to this search for authenticity and the desire to discover little-visited places.
The beaches you show are a real invitation to swim. Even if the water is rather cold at the moment, their beauty is undeniable!
I totally agree about traveling in low season. It's so much nicer to be able to explore without the crowds. But it's true that it's also a little melancholy at times, especially when you find yourself in a place that's almost deserted. I have a slight preference for Airbnb for this reason, it avoids feeling too lonely.