South Africa is fine in the summer, even the Garden Route and you might see whales at the end of August in Hermanus. You would have the massive advantage of lower prices at that time of year too. Namibia is fairly expensive and requires two flights, so it costs more. If you can stretch to it, it’s a great destination.
The Far East is a good and you could do it in budget. Don’t go wandering off to odd places on your first holiday abroad and your budget won’t stretch to a month staying anywhere nice. See below about what companies to approach.
Costa Rica would be excellent. A first class choice. We have also been to Peru in August which was tremendous. Again, budget might be an issue but it’s a great holiday for teens.
North America is ludicrously hot in the summer and gets very booked up in places like Yosemite. Las Vegas and the parks would be 40 degrees so I would avoid. Definitely an Easter destination.
My final suggestion is to look at Exodus and also Families Abroad. They are really good at chatting through the options with you and seeing what might suit your family. Their web site is full of ideas. We went with FA to Peru and it was brilliant from start to finish. Canada is also great but again your budget for 6 is possibly not enough.
The other suggestion is doing a family holiday to India. Goa is essentially beaches and not much like the rest of India, but you can do the Delhi, Agra and Jaipur triangle to include looking for tigers in Ranthambhore which is an ideal introduction to India but you really must want to go. It’s not an easy holiday but it gives a brilliant insight into a totally different culture.
Have fun talking to a family holiday expert. You won’t regret it and you’ll have a great time.