Marinus, firstly relax and enjoy the trip, it will be great! The itinerary looks pretty good to me, except for the day you leave Maun for third bridge.
I can strongly recommend taking a flight over the delta from Maun, and personally would rather do that than the boat trip, particularly since you will be boating at Kasane. However, I would try and squeeze that in the previous afternoon, by leaving Kang early or if it's not too late in the planning stage rather staying over at Ghanzi rather than Kang.
We left Kaziikini (80km closer to third bridge from Maun) at around 7am, and only reached third bridge at 3pm. It can be done faster, but I wouldn't want to. The road from Maun to South Gate is mostly graded dirt, fairly corrugated but nothing too rough. From there to Third Bridge is tweespoor with numerous waterholes etc.
One thing I need to point out is that there are two types of water in the Moremi. The vast majority of the water is from rain that falls in the highlands of Angola, hundreds of km away, and this water takes around six months to reach the Moremi. In May, water levels will be on the rise but will not be at their maximum yet. The local rain is considerably less, but can make the roads slippery. You shouldn't have any serious issues unless you are towing very heavily though. Also, take the opportunity to talk to the rangers at South Gate about the recommended routes. Finally, the much-vaunted bridges. Important to realize that these are not really bridges in the normal sense of the word, but merely logs that lie on the ground, with supporting poles to keep them in place. So you are not going to plummet to your death. However these logs do rot fairly quickly, and then there can be gaps in the bridge, and more problematically the logs can jump up and get jammed in your undercarriage. If 1st and/or 2nd bridges look bad, there are bypasses through the water next to the bridge, just make sure of the depth on the line you want to take.
Third bridge has potable water as well as solar heating for shower water, but do take enough drinking water for at least three days in case of a real emergency.
From Maun to Savuti there may be some serious water obstacles. There is a lake/dam close to Xakanaxa, and it is in this area that Kalahari Safari broke his 4.5's engine the first time, due to water inhalation. The Kwai river can also be a formidable obstacle. Once more, speak to the rangers about routes.
If the Kwai is impassable, you need to make a fairly significant detour, so best start early. Once past Mababe, I reckon you are definately looking at the Sand Ridge road. This is fairly deep sand, and if your car is auto, stick it in low range, you will save a lot of fuel. Other than that, you shouldn't have too much of a problem. Savuti now has very sturdy ablutions with potable hot and cold water. Take note that no generators are allowed, even though they run their own for several hours daily.