Thank you for your comments @chess, @amog and @andbenn.
You’ve made me realize that in the event that all my devices and onsite backups (HDD & paper) are unavailable, my Proton master password needs to be “memorable” so that I can access my backups stored on Proton Drive. But I still wouldn’t be able to access my Proton account without its TOTP (which I can’t realistically memorize). I would most likely have to store the Proton TOTP secret somewhere offsite to close the loop.
The more I think about this the more I realize how complicated it can be.