In general, yes you can use an old phone as cold wallet. Nothing wrong with that. But is kinda useless. Too much paranoia with air gapping.

A cold wallet you use it ONLY to deposit and check the balance (watch-only), so that scenario with air-gap it doesn't make sense.

Is enough to store the seed in a safe place and use that old phone to restore the seed only in emergency case or for a deposit. It takes few moments to restore it, no big deal.

The thing of keeping an old phone as a hardware wallet is just giving the impression of more security, but in the end is just an impression. Is like wearing a fucking mask. It doesn't serve to protect you.

The best way to protect you, from yourself (your own mistakes) and from other type of attacks, is to use the 3 levels stash: cold, cache, spending. Compartmentalizing your funds for each destination.