As I can see, the security options provided right now for the iOS version are password, PIN or FaceID/TouchID. I'd suggest for the FaceID option to have a PIN backup, just like banking apps etc. In case your FaceID sensor stops working, you're locked out of the app. However, if FaceID was an option under PIN/password, you would be able to still unlock the app even when your phone can't recognise your face/the actual sensor stopped working.
Thank you for the time you're putting into the app.
Diarium version: 2.9.20
OS + version: iOS 15.6.1