There are so many radios on a phone (Bluetooth, wifi, nfc, SMS, etc.)

Each one presents a potential entry point for an attacker.

Using an old phone with no connection (ever) is bad because it can't download and apply security patches.

It's better to embrace the "hotness" of a smartphone and use it as a spending wallet than to go cold with it.

Paper wallets or steel seed plates are also extremely accessible to people.

Look into seedsigner.com DIY hardware signing without being obviously for BTC.