Hi and
to the forum.
Unfortunately, I have no experience with the EZKontrollers so I don't know what benefit you would see by changing the firmware. If the 96V firmware is specifically tailored for the 96V motor, it might even make things worse.
I would try setting the battery parameters to suit your battery and then see how it performs.
The "all or nothing" throttle response is usually because the controller is using torque control instead of speed control and when there is insufficient load being applied to the motor shaft.
I haven't seen any rpm figures for the 120V 10kW motor but I would expect it to be similar to the 96V motor (~4130rpm) if you were running it on the expected 120V battery.
As you are only using a 96V battery, I would have expected the maximum speed to be ~3300 rpm
(you may only achieve 80% of its rated speed if you're running it on 80% of its rated voltage) so your 3500 rpm is actually higher than I would have expected.
As the 96V motor develops it maximum power at 3611.9rpm, I would expect the maximum power from the 120V motor to be achieved at a similar rpm on a 120V battery, but this might also be reduced to ~2890rpm when running it on a 96V battery.
This means that your gearing on your buggy will need to be high enough to achieve your maximum speed at this lower rpm.
Alan