Does the app not require authentication? At least MitID in Denmark requires a 6-digit PIN at the very minimum, or biometrics optionally.
The issue is that someone can look over your shoulder and memorize your code, if you use a pin in public.
Some people will use the same pin for screen lock and BankID, and if someone gets their code and their phone they can cause a huge amount of trouble in a very short time.
MitID (at least as I have it on Android) requires either biometrics or a PIN.
To help slightly I've set my username to a long string of random digits.
When I lived in Sweden I had an 8-digit pin and biometrics enabled, indeed.
Yes. Minimum 6-digit PIN (I don't know if there's a max) and you can enable biometrics.