If you want off-road adventure forget the 'routes'. Spend some time on Google Maps and find places you want to see. Take some waypoints and then just find your way between them. This is particularly fun in the Gobi in the south as there are fewer rivers and mountains to negotiate around.
Make sure you have plenty of food, water and fuel for this. You will find that there are car tracks almost everywhere in Mongolia, just follow them in your direction.
Pass by some sum centres (villages) or regional capitals to stock up on supplies and take a shower (if there are no rivers or they are frozen).
This was how I travelled in Mongolia for several months, and I believe it was really the best way to see the country.
Southern Route is really boring between UB and Khovd. From Khovd to the Russian border is pretty spectacular. It's worth getting farther west though, the road down the Chinese border is a real off-road slog through friendly Kazakh villages who barely ever see a traveller.
Central Route has a lot of asphalt but some good scenery and the main sites of historical interest which I would say are worth seeing.
Northern Route is also beautiful but feels in places more like southern Siberia than the classical image of Mongolia. The western end of this, around Uvs Lake is beautiful in a bleak kind of way.
But at any point you can break off from these. I took a turn off the southern route and went deep into the Gobi, saw ancient petroglyphs, and eventually reached the dunes of Khongoryn Els. One of the best journeys I have ever made. At times I was just driving across trackless desert.
In general, the West is the most beautiful and rugged, the south is the real desert, and the far east is endless grassland (great place to see huge herds of gazelle).
You can read and see pictures here:
https://eurasiaoverland.com/category/mongolia/
Good luck
EO