The primary salt is an hard coded string defined in the protocol. Using it the list of the salts used by the same passkey can be obtained from the relays. If by "the user trusts his nostr relays to store the salt", you mean to trust the relays to not delete them, trusting iCloud/GDrive is not better. Also, Breez intent is to mitigate the trust by handling a relay dedicated to store the salts.