Sorry, to clarify, you need to keep a backup of all subkeys along side your master/certifying key. Data encrypted to a given subkey can only be decrypted by that same subkey.
Honestly, if you're using GPG strictly for personal password wallet encryption, and don't intend to maintain an identity tied with the key long term, there's no need to do the subkey thing -- just create an offline key, load it to your YK, then securely store the offline key.
Subkeys shines when you want to maintain an identity long term, while allowing rotation of the keys that do your day to day encryption (or signing/auth)
Honestly, if you're using GPG strictly for personal password wallet encryption, and don't intend to maintain an identity tied with the key long term, there's no need to do the subkey thing -- just create an offline key, load it to your YK, then securely store the offline key.
Subkeys shines when you want to maintain an identity long term, while allowing rotation of the keys that do your day to day encryption (or signing/auth)