I understand that you want a PIN on your banking app as a separate thing, but are you using iOS or Android? There are probably third-party locking apps you can get for Android if you need this feature today. If you’re on iOS, though, you can use “Content & Privacy Restrictions” in the screen time app (or on an older version of iOS, parental controls or restrictions) and restrict Monzo specifically, then requiring a separate passcode to disable restrictions from the normal phone PIN.
I understand that this is way non-ideal, but it’s better than nothing if it precludes you from using the entire bank because of it.
I would never in my life have thought I would suggest a Norton product, but you could try their app lock app. The benefit is that you know for certain the company isn’t a super sketchy company doing something evil with it. Android and iOS are both really locked down, so it’s harder and harder for malware to exist (but not impossible).
