An old mobile phone makes the best cold storage wallet (i.e. signing device)- change my mind

I keep considering a dedicated bitcoin hardware wallet, but I keep coming back to using an old mobile phone that's encrypted at OS level, permanently in aeroplane mode (no SIM, WIFI networks deleted), and running BlueWallet with encrypted password.

Here's my logic:

Once the phone goes cold (i.e. radio disabled), I create a wallet in Bluewallet (or Electrum etc.) and can then use it as a signing device via QR codes (i.e. fully air-gapped).

• it's cheap,
• I can audit the Bluewallet source code, compile and install it myself,
• it's encypted (by Bluewallet, and also possibly OS-level)
• it's fully air-gapped (no need to connect it to an internet-connected PC)
• to anyone who finds it, it just looks like a old mobile phone - not a digital treasure chest that's worth days/weeks of hacking

What am I missing?

Update:

The conclusion I've reached is that, while it's very likely safe, while the phone is running proprietary software (OS & BIOS) you can never be 100% sure it won't randomly connect to a cell and be exploited such that it will transmit your keys.

I'm yet to see evidence that a factory-reset phone, that's never connected to the internet and had aeroplane mode enabled can be infiltrated by malicious parties - but there's always a small chance it might happen.

I've also come to conclusion that I don't need to trust the SW running on a dedicated HW wallet providing I keep it air-gapped, generate my own seed phrase (e.g. physical dice) and use open-source SW for generating and publishing the transaction.

So I now believe that if you have access to a HW wallet that allows the steps I mention then that's the best way to sign your transactions and that's what I'll be doing from now on (I've chosen the seedsigner)