microcontroller current supply - noise minimisation
You'll need to isolate the microcontroller from the motors. This can be easily done with a separate battery on a common ground, which it sounds like you're not eager to do, or by a diode and a sufficiently large capacitor on the microcontroller side. You'll want a small capacitor for small line noise and a larger one to cover brief dropouts from battery spikes.

Be aware that a diode will drop the voltage slightly between the battery and microcontroller, so you'll want to account for that when selecting the needed capacitance. A schottky diode will help minimize the drop.