Electronic – Undervolting BLDC motor on electric bicycle

I think you misunderstand what "controlling by voltage or current" means.

The motor, as a matter of fact, is always controlled by voltage, a bit like a car is controlled by fuel injection. In the case of a BLDC, we usually talk of a modulated voltage generated by a voltage source (let's say 24V).

And although it is true that voltage is associated to speed, voltage induces current, and current relates to torque. What you usually need to control at your lowest level is current (or almost directly torque).

Controlling current "directly" may mean to switch the voltage measuring current and comparing it to a setpoint. Have a look at keywords such as DTC (direct torque control). This method is harder to master, and more complex to understand and implement, but it usually results in better performance.

A simpler, more intuitive way of controlling the motor is by making use of a standard PI controller, employing the Direct-quadrature transformations to transform 3-phases (I assume) currents into quasi-linear currents Iq and Id, where controlling Iq has a direct influence on torque, while Id should be kept at 0 as it's associated to energy losses.

Back-emf should be compensated for a good torque control.

Frequency goes up with velocity... Careful, because with it, also disturbances get more nasty.