If I want to create an oidc identity provider that uses a custom authentication/login flow (in my case via an ethereum wallet), what would be a good place in the Ory ecosystem to start from? Would modifying/extending/using parts of Kratos be a good idea? Would I also need Hydra for storing the custom identities?

Hydra would be your oidc provider. It would basically wrap whatever auth flow you have in an oidc compliant layer. Kratos would be to do user management, password reset, confirmation, etc. Hydra does not store user info, kratos does.

Curious about your use case! Are you looking to use ethereum addresses as a unique identifier to then have your service be an oauth2 provider for them?