This is definitely something to be aware of. Installing this firmware definitely invalidates the StVO (though I can always flash the original firmware back). Personally I am morally sound with replacing the firmware of the display unit as the critical functionality is still handled by the motor controller (for example, the brake lever sensors are wired to the motor controller directly, and the display must report the throttle position constantly to not trigger a shutdown); the display unit would need to contain intentionally malicious code to cause problems.

Maybe, but in the case of a crash, would anyone go Ghidra-ing inside your scooter's MCU to check if you replaced the firmware?

It sounds like as long as you don't say anything, no one will ever know.