Google Photos has a feature for exactly this on Android. It's called "Locked Folder". It is not synced, and accessing it requires your screen lock code or biome

Google Photos has a feature for exactly this on Android. It's called "Locked Folder". It is not synced, and accessing it requires your screen lock code or biometric authentication. You can choose to save photos directly into the locked folder as you take them, in which case they are never uploaded.