The roads in to these areas can be very bone jarring. A second scenic area is the Air Mountains, near Agadez this is even more rugged riding. You will generally want to stay on prepared roads because there are some very trouble thorns in the brush. Most likely you will be riding along Route National. The national highway from Niamey to Zinder is relatively flat, paved and in good condition.
Traffic volumes are so low it is nearly stressless cycling.
Though the best part of your trip will probably not be Route National, but the side trips. The area out of Tahoua to Keita and Tchin Tabaradene are culturally mixed and fascinating. From Birnin Konni to Zinder there are a number of interesting small towns. In general my favorite days in Niger have been spent at smaller towns on the weekly market day. As you travel keep asking about weekly markets in the area.