Richard, that should be a great roadtrip, and you wife has to get a reward for being cool with you flying solo!
Not sure if everything is booked and paid for yet, but be aware that due to the drought the Victoria falls
have all but dried up, and as our dry season is now starting you are not likely to see more than a barren dry cliff - might not be worth your while, honestly!
Botswana is great though, and you are guaranteed to have a really good time!
May is an ideal time for game viewing. This is mainly as it is mid-winter and therefore not quite as unbearably hot as you will experience from November through February. The key benefit of this is that animals don't rest in the shade as much by day and move around a lot more for grazing fodder. Also because the foliage has typically died back significantly by this time there is less place for them to hide.
If I may suggest you consider stopping over at the
Mapungubwe National Park on your way. Lies where the borders of
South Africa, Zimbabwe and Botswana come together. I know the Park Manager very well, and he will give you the "non-public" experience guaranteed!
Over and above the game viewing aspect it is also worth noting that Botswana is absolutely a meat eater's paradise. The meat is of fantastic quality and really dirt cheap, especially as due to local custom there is no demand for boneless cuts such as rump, sirloin and fillet!