First, test all three throttles in the same way. Chinese quality control is horrific.

Second, get that controller into cooling airflow.

Need accurate measurement of top speed with freshly charged battery. "20-ISH" is not good enough. 18mph, 20mph, 22 mph. Need precision here. Two of those numbers indicate a serious problem.

Do you have a speedometer installed?

That motor will pull a lot of amps. It will pull more amps when accelerating from a dead stop. It will pull more amps if the phase wire connection is out of sequence. High amps can melt phase wires, also cause transitory sag triggering low voltage cutoff on a weak battery like yours. High amps will also overheat a controller.

Does your meter show amps or watts?

That motor with that battery should not be engaged from a dead stop. Pedal only to at least 3-5 mph.

Find the phase wire test at ebikes.ca. It is in the troubleshooting section. There should be two, separate tests, one for a shorted phase wire and a different one for correct sequence. Run both. Carefully.

Both of these errors are quite common. Incorrect sequence is extremely common and often difficult to detect. Lowered range and lowered top speed are two primary indicators, as is melted phase wires, particularly twice.

Preferred connector is XT60 or Anderson. Installed bullets will NOT survive a high number of disconnection cycles.

The motor type itself is quite reliable. However, IMO you bought the wrong kit for the type of riding and particularly the battery you are pairing it with. Either a slower wind motor, a small geared hub, or a stronger battery, in either amps or c-rate.