I would really advise you go with sand option, as your base, on top of the base you already have, no disrespect to you but as builder said about getting your level with concrete is not that easy, If you've not had previous experience, and you doing it in little bits, you may end up with lots of different levels. The sand base, along with a compactor plate is a lot more forgiving for someone who has not done this before, if the level is to bad you can just re- rake it up again and restart level with compactor plate, with concrete once its goes off you stuck with it. You can still do a smaller area with the sand base, albeit not ideal, but it don't take long to shovel sand down and spread it then compact it down level; and if weather turns bad/ wet, you can just restart when weather improves.