I need some help with cold storage!

Hi everyone! I plan to set up a cold storage wallet and I would like to hear your opinion if described method is safe enough.

I have an old phone which I would use to download blue wallet, disconnect it from Wi-Fi, turn of my Wi-Fi router, generate wallet,write down seed phrase and import my public address to my new phone for watch only wallet. After that destroy my old phone and store the seed phrases. I’d use this wallet as my vault, only to store bitcoin. So only depositing, no other transactions whatsoever. Then when and if time comes to withdraw I’ll import the seed phrase and withdraw. Is this considered safe enough? This would be my retirement wallet so I don’t plan on using it for a long time. Thanks!