I ended up taking the QL12 national road followed by the QL4D national road as researched before.
I was hoping to do that journey by bus by putting the motorbike in the bus (yes they do it usually!) & get some rest in the bus. But they did not want to do it, so had to ride again & for a tougher ride this time.
The road was really bad. It was very neglected compared to the other roads I took before (Hanoi to Mai Chai to Son La to Dien Bien Phu). From what I remember, maybe 50% of the journey was terrible (narrow & many times very damaged). This could get a little scary in some places, so ride carefully.
I think it took a total of 6h (with 1h break).
Lai Châu is a very funny/strange place. It has epic avenues, it is very spread out & has big buildings (governmental I guess) but there is not many people who seem to actually live there. A place of interest if you want to see the weird side of Communism in Vietnam I suppose.
I bumped into a Spanish blog having an article about the "Dien Bien Phu to Lai Châu" 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 "Dien Bien Phu to Lai Châu"" GPS track on Wikiloc.