You don't have to do all that, actually that would be less secure. That old iPhone could be stolen and then compromised. The app could have vulnerabilities.
Why not just encrypt the seed using basic text encryption with a password and writing the encrypted string on a paper instead?
It's basically what you're trying to accomplish but without any concerns of hardware failure.

Notepad++ allows you to encrypt text using different algorithms directly from the UI using a plugin called NppCrypt.

...Or you could get a hardware wallet.