so one thing I forgot to mention, is with field weakening on a pm motor, since you can push it well past the pack k/v limit, you can't just shut off field weakening at high rpm.
Also I don't know how suitable it is for a leaf motor, but a somewhat common thing to do with induction motors is to reconfigure them for a lower voltage, i.e. open it up and switch it from wye to delta (sqrt(3)), or reconfigure the poles in each phase to be in parallel (div by 2 or whatever). Occasionally folks will rewind them for lower voltage too, but that is a little involved. And a second hand leaf pack is like $2000, so hard to justify except in space limited scenarios.
so paralleling fields increases the k/v by two (depending how much paralleling you do, depending on number of poles). but reduces the k/t by two, so you need twice as many motor amps for the same torque, but you doubled the number of conductors so it should be ok. With those constraints and a suitable battery/controller you can get back close to the original graph at a different voltage, though switching losses will be greater. rewinding has a similiar effect, increases conductor cross section and reduce number of effective turns, only a bit more precise about matching to a specific voltage instead of 360/2.