How to keep private photos off your camera roll
Take them out of Photos
The Hidden album still lives inside the Photos app. Memories, search, and iCloud can still see that library. A separate app is a harder boundary.
Give the vault its own lock
Mavaul uses Face ID or a passcode that isn't your phone passcode. You can also put a calculator on the home screen. It does real maths. The passcode typed on that keypad opens the vault.
Leave them on the phone
AES-256 on the device. No account. Nothing uploads unless you turn on an encrypted backup to Dropbox, Nextcloud, or Files, and that file is encrypted before it leaves. The details are in Privacy.
Sharing
Wi-Fi Share opens the vault in a browser on the same network. The session has its own code. The photos don't go out over the internet.