I have one too. I've gotten all sorts of things inside the inner screen. Once I even accidentally closed it with a key in it (basically folded it while putting it in pocket where keys are). Realized only after arriving home and pulling out my phone with key in it. Dust, manufacturing materials (I work in a factory), ectera ectera.

What has saved me all this time is simply a nillkin case that has a hinge protector (dust, falls, etc.) and creates a small gap between the screens when you close, so whatever dust gets inside is never really pressed between the screens.

I went with OnePlus Open. Samsung had weird screen ratios and pixel was just meh in comparison.