I did the new highway on the RoB last summer. The new road is pretty easy, with minimal water-crossings; the Old Summer Road is pretty weather-dependent and I wouldn't try it by yourself, as it is pretty remote.
The Lena Highway, from Yakutsk to the main Siberian highway, is about 1000 km of gravel road. Not technically difficult, but pretty unpleasant because it is long and boring and can be very dusty, etc.
If you want an easier ride you can just stick to the main Siberian highway from Vladivostok.