Hi,
4x4 in Maun? It's hard to get them there. Most Tourists fly to Maun to continue to Lodges.
I just came back from Botswana, however started and ended in Johannesburg. I rented from Bushlore which was very good in all. I also know that they will transfer 4x4 where ever you want them, and know that they are opening (for various reasons) a depandance in Kasane. So you they will bring you the car to Maun if you want it there.
In 2008 I spent 2 nights in Grassland, and I enioyed it. Well on the way therto you will have to open and close about 17 gates. What I really enjoyed was the walk with the bushman. Yes they have lions and wild dogs, which is fine and nice. To see them in nature is much better. But the walk was really good, better than the one I did in Hainaveld.
Why don't you take over the car in Johnnesburg, 1 night Johannesburg where you can stock up, next day drive to Khawa (we did this in september in about 8hours) short game drive, than via Rakops to CKR (or if you don't want via Maun to Grassland) If CKR 2 nights CKR, leave via Tau Gate and skip 2 nights Ihaha Chobe. I agree with the others If you stay on the boat you will see a lot of Chobe.
In Moremi I would skip North gate and stay at Mohotlo (cheaper nicer better) and drive direct to Savuti (with Sankuyo you are driving backwards again). Stay one or 2 nights in Ihaha, drop the car at Kasane and continue your tour.
Stefan