Milos and Santorini: how overtouristed are they really?

Bear with me. I always set myself long-time travel challenges, and after just having visited two active volcanoes in Italy I want to visit all active vocanism sites in Greece belonging to the South Aegean Volcanic Arc: Santorini, Nisyros, Gyali, Milos and Methana, with maybe dormant bonus points for Poros, Aegina, Sousaki, and Kos.

I do wonder if hiking and geology is the main interests, how bad are Santorini and Milos really? Would probably be staying a week on each island in the end with a rental car and a holiday home centrally located (not sure about Gyali, maybe as a daytrip or 2-3 nights), thus really a long-term project.