I'm just following up on my question now that I experience myself some answers.
After I decided against going to Gobi and ended up with extra time on my hands when I reached UB so I changed my mind and headed to the Gobi desert.
To Dalanzadgad is an 8 hours ride, all good pavement except for the 70km after Mandalgovi, that has construction detours and big potholes.
From Dalanzadgad to Bayandalai, 100km is also new pavement. In that way you have the Yolyn Am Canyon, very beautiful.
Once you get to Bayandalai is all dirt, with the typical tracks in Mongolia going from one road to multiple ones and then merging again.
To the tallest dunes is 120km in total. The conditions are rough but not a big deal, just short sections with a bit of sand, some mud, small rocks sections, and a lot of washboards but not deep sand or river crossings. Although, I crossed several dry river beds, this was end of June, I don't now if at any other time there is water on them.
There is small gas station in Bayandalai just right at the beginning of the town on a dirt road to the right coming from Dalanzadgad (the other one is abandoned)
Please put some gas, I have more than 400km range, didn't and ended up siphoning from a car. The bike used way more gas on this 120km than on average riding. There is no gas at all after this town on this road.
The important part is to take the north road, so you get to the dunes with them on your left. I believe that the south road the one that lead to Servrei is the one reported as difficult (Didn't ride it myself I'm just speculating)
The experience is fantastic and probably is what I liked the most in Mongolia.
I have some details and pics on my ride report in advrider.
https://advrider.com/f/threads/madri...378256/page-10