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.
I already have a Trezor hardware wallet, which is secure-ish in itself. But I'm talking about the backup seeds, which I filled out on a card. But the backup card is as good as cash if someone finds it - is it not?
Yeah, the app might have vulnerabilities. But even the FBI has trouble getting into iPhones, so why would that not be more secure is what I'm asking? I guess the real question is if I keep the phone offline from now on, nobody can hack it through the internet, and if someone physically gets it, it would still be harder to get the seeds than if someone physically gets my written down seed card.