Optional is fine, I'm not looking to force anything on anyone else, but as an Android user without a fingerprint reader, there's no option.

I can speak from experience about user perception of security, having previously run security product for a major UK fintech (which works with Monzo). A large proportion of users would complain that biometrics and/or PIN was not mandatory when opening the app. Many people share devices, especially iPads, or have family members that know their device passcode (or have a registered fingerprint or face).

Making it mandatory (or strongly pushing it) greatly increases the level of trust people have in your company. The data suggests it is a worthwhile convenience trade-off.