I don't know this specific area, but have enough MA and New England time that I have a hunch 20 is your best bet. From a glance at the map, I first thought something more residential / country clubby like Monson Rd would do. But Google voyeurism confirms that is a little two lane shoulderless road that I bet people rocket down. At least you have a shoulder on 20.

You might want to try a test run on Saturday or Sunday to confirm the route, scan for potential trouble spots on a day when there is little traffic and you are not pressed for time, and get a sense of timing. I'm sure you know it in a car, but it is different on bike. That way when you dive in on your first commute day, you have a little less uncertainty.

Bon voyage.