The bright aluminum parts of my CX5 2018 rims

Dark stains.
show dark spots. What can can be done to prevent further dragen or even better to cure the problems?

 

You could try using a polish like Flitz, then sealing with some wax or polish. It won't remove all the corrosion but it will likely reduce the amount of visible corrosion. Hopefully, most is on the surface and has not penetrated too deep.