You need to have corresponding battery to your motor controller firmware. Firmware can be easily changed, battery not so easy.And it works well? Isn't there a problem with the Cut Off?
Battery 43V Cut off = 40.8V ( 3,4 V / cell)
Battery 36V Cut off = 34V (3.4V / cell)
Intuitively, I'd say that with a 36V you have an operating range that goes from 42V (Full charge) to 40V (Cutoff) and then it cuts off, right?
Working range for li-ion cells is about 4.2-2.7V with most BMS. So 36V battery would be 27V to 42V.
If you like an extender and don’t want to build your own batteries, go for 36V or 48V. 43V seems to be Bafang specialty and finding an extender might be difficult.