In/on an island

Hello everyone,

Which preposition (in or on) would you use in the sentences below?

- I spent the summer on/in Thassos.

- Next year I will try to get a job on/in (one of) the Greek islands.

- He worked at a hotel on/in Corfu.

I am a little confused because I've heard people use both "on" and "in". I would definitely say "in Cyprus", but I am not so sure about Thassos, Corfu, Kos, Paros, etc. as they are part of Greece.

Thank you.