Out of curiosity, what's the challenger bank you started? I'd love to learn more.
I'm using N26 right now, but I have since stoped using closed-source software altogether on my phone, and will likely switch full-time to a pinephone. One of the things that irks me is that multiple services require 2FA trough their proprietary app or SMS rather than TOTP or FIDO/Webauthn/other security-token based tech. Not sure if that's for legal reasons? In any case, that's my number one usability issue with other banks.