You’re correct, the Eurotronic Z-Wave isn’t a stepper motor, it’s a standard motor & gearbox with regulation through current detection. As I’m sure you know, it uses this to sense the top of the valve pin in a fully open position and then the closed position during initialisation. Small changes to the TRV body, such as tightening the mounting or removing and replacing the TRV, cause errors and the device will need to be reinitialised (makes sense due to the detection method, I suppose).
If you disassemble the unit you’ll find that there is a small window into the gearbox and an IR detector on the PCB which monitors the rotations of the main gear - a rudimentary rotary position sensor. For this reason the resolution of the device is poor.
It’s designed to suit many different types of TRV heads and hence tries to mange the valve over a wide actuator range. Some TRV valves are “short-throw” (~5mm) and some are “long-throw” (~10mm). Are you sure that your valve is maximising the actuator’s range?
That unit is also known for over-shooting in terms of the internal PID control. I don’t find it too bad, but it can remain open (~20%) when the sensor is >2 degrees C above the setpoint. Annoying.
From what I have seen, Eurotronic have not been very engaging with customers/opensource in terms of product issues and fixes and for this reason I’m not buying their newest Comet WiFi TRV.