The motors that use different windings for different speeds will have more than two wires connecting to them. . Or that is how I would identify them. Also they would have no resistance coils in them.
so obviously you do not have a tapped type winding motor of some form. A rheostat will work but it has to handle 25 amps of current. That is a substantial rheostat I suspect. I did not bother to look up or price one.

You start changing direct current to some form of chopped up waveform with an electronic controller may be hard on the motor brushes and could cause motor heating. May not yet could.