This is a wild theoretical concern with no practical attack. Nobody knows if, with a horde of encrypted keys, you could somehow hack them better than if you were just trying to go after one. If there is a good reason to put them online, that might easily overwhelm this kind of excessive safetyism. They should be very secure. Not only because of the good and excessive crypto (xchacha20, good cryptographers are now saying 8 rounds was enough, 20 is crazy) but also from the intense key derivation (scrypt, maximally memory hard) and further because the plaintext is both SHORT and virtually RANDOM.