Power (W) is A x V. As battery depletes, power goes down so with 1/2 battery bike will not have the same power as fully charged so there is nothing we can do with it. Google NMC discharge curve, it is not so bad, middle part is almost flat and lasts long.
Controller has settings for 1st and 2nd gear.
Then there is also undervolting protection and a limp/turtle mode. I am not sure that it is possible to change these values, I am to lazy to go to bike and check it

Bars on display are not precise, do not trust them.
Voltmeter is a good thing, you will see voltage sag + actual voltage to be able to determine when to run to the charger.
I am affraid that there is not much we can do about battery.
If I did not understand what you mean feel free to kick me.