If you’re paranoid about your 24 word seed being compromised, would it be safe to put it on a pin protected iPhone/Android that you never take online?
It doesn't make sense to me to store a 24 word seed on an (always offline) mobile.
If your goal is to have a full offline backup, why not store it on paper? Or an USB-stick / SD card ?
There is way too much 'technology' in a mobile to be 100% sure its completely offline.
Especially for backing up information in form of 24 words.
If you want to increase the security of your offline backup, then use a simple storage device and encrypt your seed phrase with a pin/password.
Even better would be a (correctly generated) paper wallet or a hardware wallet. Those two would give you the 'highest grade' of security.