Some other email not directly linked to you, as you said. Second proton mail account, linked to a temp email, for example? Buy access to some questionably sourced email owned by a random person, and use that as a recovery email for that secondary ‘secure’ account .
Although if security is important, I would probably just not use a recovery email at all. Back up communications to a ‘secure’ location directly, and leave the account itself to die if you forget a credidential.