Exhausting but great ride through the Usambara Mountains. The route leads up to almost 2000m, through forest, fertile valleys and many small villages. I would not cycle the rout in the reverse direction as the climb in the west is too steep to cycle with a loaded touring bike.
The road to Lusotho is tarred and a nice steady climb at 5-6%. After that it is a dirt road that get rougher the further you get. Before Mambo there is quite some bulldust (ankle deep fine dust). The downhill after Mtea is very rough and steep. I would not try to come up here. The track back to the main road is again quite good.
Between Maun and Kasane there are not many accommodation options. There are also only very few villages. If you don't want to camp in the bush you have to plan a little bit so as not to end up in the middle of the route. Personally I would not camp in the bush here, as there are too many animals (elephants, lions) along the road. Elephant Sands is a great lodge with a water whole, where the elephants come in the evening to drink.
In Lusotho there are guesthouses and in Mambo is the very nice Mambo Viewpoint Lodge where you can camp.