I know someone that tried and failed to regulate their trolling motor like that. The regulator didn't help any to lower the speed between selector switches. I think he finally got it to work when he too out the selector switch and rewired it but still didn't have any control of the first step up. Guess the motor was simply not meant to go slower than the first step.
Only thing I can think of to get a slower speed any other way would be to get another prop in a different pitch.
I love taking my kids fishing, now if I could just manage to fish at the same time.