Here is my input after doing it myself.
It's completely doable, the ride is pretty easy actually.
It takes a total of around 5h: about 4h ride, plus 1h for stops on the way (going to toilet & taking pics).
I took this exact journey (see itinerary on google map):
First, I left Hanoi without taking the highway as it's forbidden for motorbikes.
Then, as seen on the google map itinerary, I headed south at a junction about 10km after Xuan Mai (through a small town called Xóm Vè). This is the back road mentioned in the article I talk about in my question. It had indeed much less traffic, the road was just fine. I was not blown away by the scenery but it was definitely nicer than to be on the QL6.
Finally, joined back the QL6 & carried on until Mai Chau.
I bumped into a Spanish blog having an article about the Hanoi-Mai Chau route. The nice part is, they shared the GPS tracking for the journey at the bottom of the article. You can also directly find this Hanoi-Mai Chau GPS track on Wikiloc.
Make sure you leave early (latest 8am) so you have plenty of time in case of a break down or other issue.
Leaving early will also make you much more relax, many positive things come with this mindset.
Finally, it means that you will arrive early enough to pick your accommodation during the day time. So again, less stress, find a place to stay that you really like (location, comfort, price).