Your idea isn't bad but I think you're too paranoid about your keys.
Stainless steel and engraving on it the keys and have a copy like 3-4 pieces and place it on different places is enough to protect your private key.  It's fireproof and waterproof or any disaster will come your private key were still there not unless someone will steal it from you physically.
Your book is also good since it has a strong cover, place your keys on the different pages and spread them, for sure no one will suspect them as your private keys.

Saving your private key offline is enough, just buy hardware wallets it has better and safety features that can protect the hardware wallet itself, Trezor and Ledger Nano S as an option are enough too.