I am guessing the Developer Options variable you are referring to is “OEM Unlocking: Allow the bootloader to be unlocked”? This doesn’t actually lock a device, instead it prevents the fastboot command to unlock again in the future.
To re-lock an existing unlocked install, using fastboot flashing unlock is needed, but note that locking (or unlocking) will wipe out your userdata partition, effectively performing a factory reset and all your personal data will be gone.
Regarding the banking issue, if you have all microG options enabled including all “Self-Check” options, “Google device registration: On”, and “Google SafetyNet: On” then I believe that is the best that can be done without full Google Play Services. Sorry.
For N26 specifically, there are sadly “broken” reports from Plexus on it for Android 15 based ROMs with microG (I think you need to use the Plexus app to see more details which showed me this).