There is no "perfectly secure method" for storing private keys and seed phrases (much like cash or bank passbooks).
Below are common storage methods, each with its own advantages and disadvantages.
1. Cloud Storage (Google Drive, etc.)
Pros: Highly convenient, as it allows access from any device.
Cons: Carries a risk of leakage if your cloud account is compromised or hacked.
2. Local Device Storage (Smartphones / PCs)
Pros: Relatively secure when kept in an offline environment.
Cons: Recovery becomes extremely difficult if the device malfunctions, breaks, or is lost.
3. Password Managers
Pros: Highly convenient and securely encrypted.
Cons: If you lose your master password, you will permanently lose access to everything stored inside.
4. Paper / Physical Notebooks
Pros: Free from digital risks like hacking or malware.
Cons: Carries physical risks of being lost, stolen, or damaged and deteriorating over time.
Limiting your backup to a single location increases the risk of absolute loss, while storing it in too many places expands the potential pathways for a data leak. Carefully weigh the balance between convenience and security to choose the method that best fits your management style.