[pilleya]

Unfortunately Window Motors aren't permitted in FTC, some window motors are a fair bit more powerful( thus more current draw) than the NeveRest 550 size motor so the FTC electrical system would probably hate you for using one anyway.

You could try and make some 3D printed parts to change out the Window Motor's motor for a Neverest motor, it is a fairly similar output RPM but will probably have a bit less torque. Probably won't be very easy though.

Alternatively you could use a VersaPlanetary in combination with a NeveRest, allowing you to fine tune the reduction and output speed which should improve the smoothness( gearbox RPM won't really change as the load changes if it is geared correctly).