Good idea. Use the $39 Commander and see if the engine achieves an acceptable speed when pulling the desired load with 14VDC or whatever max voltage you can provide. I believe it will run faster than when you varied track voltage to control speed (i.e., when not in DCS command mode).
Yes, in principle you could bypass the DCS electronics and directly drive the DC motor from DC track voltage. You will gain top-end speed capability because this bypasses the engine’s internal bridge-rectifier which eats 1-2V. If you ultimately go this route, note the engine might move with, say, 5V DC on the motor. But the DCS electronics won’t operate at that low a voltage. So when you slow or stop the motor (engine), sounds and lights will turn off. I’d find this a show-stopper but that’s just my opinion. There would be other odd or head-scratching behaviors as well.
