The power drops as the voltage battery drops, i think there i not many options, as far as i know, you could improve the size of the battery or also with good quality cells suited for higher discharge rate, for example molicel p42 2170 or similar, that way there will be less voltage sag. You could also use a motor with lower kv, lower kv means more torque at lower speeds, so it doesn’t need to go high on revolutions so voltage will be just a bit less important.
With a vesc controller there is many options available to configure your setup, you can play with the acceleraion curves, field weakening or MTPA algorithms.
Unfortunalely there is no such an option to level out the voltage in vesc, i think it is more battery related.
i did used in several projects, no foil yet but soon, the vesc little focer v3, i dont know yet for this application but maybe it is a bit overkill?? it’s up to 20s and 250 phase amps.