Nice to hear of someone trying an adventurous route rather then the boring Southern Route!
I did a similar route to what you describe, but in reverse.
Read about it here:
Stage 22 – Russia & Mongolia: Altai To The Gobi Desert [3/3] | EurasiaOverland
I was in a 4x4 with a friend. We left the main road a little bit west of Arvaikheer and drove on good dirt tracks to Bogd, where there are very nice camping spots near the river.
South of Bogd it was a bit tricky to find tracks over Ikh Bogd Uul, but eventually we found a good route up onto the ridge, then continued south to Bayanlig. Here there is a good place to eat and a public shower.
From Bayanlig we made a trip to see the incredible petroglyph art at Bayangiin Nuruu, but this was a detour, and I returned to Bayanlig afterwards.
From Bayanlig there were no real tracks east, we just drove across the desert, a really wonderful trip. Khataan Suudal Volcano was a great landmark to stay on course.
Beyond Khataan Suudal the terrain got pretty sandy. Towards the western edge of the long valley which contains the Khongoryn Els (dune field), we were stopped by deep sand, and turned south, climbing into the mountains to Sevrei. It might have been possible to stay north of the dunes, but we didn't want to risk it as we were starting to run low on fuel.
We then dropped and crossed the els (dune field) in a sandy valley (after waiting for floodwater to recede). We spent two days camping at the dunes and it was perfect.
From Knongoryn Els, the track to Dalanzadgad is good, and easy to follow, nothing to worry about. There's a good food stop in Bayandalai.
I would say this is a very doable track, but have a look at google earth first and plot some routes and waypoints in your GPS so you have some reference. We only saw one other vehicle between Bayanlig and Sevrei.
No river crossings. Initially a lot of mud in the valley between the two sides of the els, but we waited a couple of hours for the water to drain, and it became OK to drive on. If there is no recent rain, there will be no mud.
Sand? yeah, lots, the fun is navigating around it. Make sure you have plenty of fuel, food and water!