DC motor parameter identification using microcontroller
The best design will have a current shunt options for 100 mV max draw at 10x rated current, to represent start current , I=V/DCR of motor coil. Selectable shunts in the &lt; 1 ohm range may be necessary for a wide range of motors using NFETs on the low side or if you can manage a good low noise Diff Amp with selectable gain to 5V max using analog switches.

Then to measure RPM for accurate speed you need some sensor, either a tach, or a type II PLL with a tracking filter to derive the RPM spikes to a 1 shot tach cct.

Max Hp load at rated voltage will be approx. 82 to ideal 84% No load max RPM rated in kRPM/V.

Current feedback average will be your torque feedback and multiplied by RPM gives mechanical power. Of course losses are many TBD that determines efficiency which requires an understanding of rotational kinetic energy and how to measure it, which you did not mention.  One can use a variable shunt resistor using a PWM NFET on a generator  to load a motor, coupled by 8mm belt drive common in mini CNC stores.  The shunt dummy load can be a halogen light bulbs of suitable cold resistance such that the RdsOn of the NFET is much lower for cool operation and is close to matching the power resistance to brake or stall the generator by uC control with a sweep of duty cycle.

You can go simple or comprehensive in your motor eval. by adding a temp sensor IC to the motor ,thermally insulated on the outside.