How about: store the otp secrets as well as the backup codes in an encrypted file/hashicorp vault/keepass (not with other passwords)/Authy, with the encryption

How about: store the otp secrets as well as the backup codes in an encrypted file/hashicorp vault/keepass (not with other passwords)/Authy, with the encryption keys being stored on USB flash drives and yubikeys, which can be put both on your keychain and in a physical safe. Many possible variants of this for different use cases.