If you have upgraded your cell phone then it's better to place your wallet on old cell phone which is no more required and keep it offline. This way you can have an air gaped device with zero investment.
Using old phone as cold storage is not a new idea, I've been eyeing it too in the past. And I got convinced that the minuses of such an approach are worthy to be taken into account, hence I've abandoned the idea. Maybe it worth reading this topic:
https://bitcointalk.org/index.php?topic=5377997Your seed is your wallet. Lost your seed, lost your coins. There is no way to recover your coins in case your seed is compromised or lose your seed. We can say that's one limitation of Bitcoin
It's not a limitation or bug. It's a feature/it's by design: without this you don't have proper trustless self custody for your money.