I don’t know if what I do is the right way around this but, as stated Proton will reject disposable verification emails and you cannot use another proton account to verify a new one.
My workaround for this is to verify proton with a Tutanota account which is also created with as little to no identifiable information as possible.
TLDR: Proton accepts Tuta emails for verification and Tuta emails can be created anonymously.