This sounds like a great adventure - quite a distance to travel in 2 weeks, but certainly possible. Several years ago, we took a 17-day small-group tour from Cape Town to Victoria Falls, via Namibia (Sossusvlei dunes, Waterberg Plateau, Etosha) and Botswana (Okavango Delta, Chobe), with one night in Cape Town and two nights in Victoria Falls. There were several long driving days through Namibia, which is mostly vast empty desert.  (Travel from Cape Town to Sosussvlei covers about 800 miles.) We had scheduled extra days in Cape Town before the tour because one night is simply not enough for that beautiful city, making our trip closer to 3 weeks.  You could manage the same itinerary within 2 weeks by eliminating the more distant Etosha and Waterberg Plateau in Namibia, but I recommend including Chobe NP as it is on the route between the Okavango Delta and Victoria Falls, and has a stunning array of wildlife in a relatively small area. The only way to reach the Okavango Delta is by air (bush flights) from Maun; lodges in the delta generally arrange these flights to small landing strips near the lodge.  There are reasonably-priced lodges in all of these areas, but they do not advertise heavily.  (In remote areas the lodging rate often includes meals, as there are no restaurants nearby.)

It is possible to travel comfortably on scheduled bus routes between major cities in southern Africa; but reaching Sossusvlei and Maun independently will be a problem. You could fly to Windhoek and take a private tour to Sosussvlei, then possibly fly to Maun via Gabarone. Air pass schemes that offer good rates on flights within Africa, some in conjunction with your international flights.  Since you want to travel independently to a few locations in different countries, your best option may be to find a tour company to arrange a custom tour for you - either a US company or one in South Africa, which has the most experienced tourism industry in the region.  Specify that you want to stay in mid-range lodges. (Sorry I cannot recommend a company, but hope that others can.)

We visited in August, late winter there. The weather was pleasant – cool at night and early morning, but pleasantly warm mid-day and not a drop of rain. Foliage is not overgrown at that time of year, better for spotting animals. The only negative aspect was the shorter days – sunset before 7PM and sunrise about 7AM in August, better in September.  The South African climate reminds me of southern California.