I have looked into brushed motors (shown as copper-coloured lines) since their control requirements are much lower, and the CL-1642M motor is the most ideal assuming that the resistance of the coils can be calculated as [rated voltage] / [stall current]:

The reason why it's ideal is because it has a 2mm dual shaft, making it straightforward to add both a magnetic encoder and POM gear, as well as using the lowest power out of all the commercial offerings.

The main downside is that the supplier doesn't sell samples at all.