While I do agree that there is an overall increase in the number of "international" schools", I think the bulk of the increase are in lower tier schools (like the ones you are referring to in TIE). These schools are catering to the lower middle class group and thus, charge lower tuition fees. There will always be a market for upper tier schools and families with the money will always pay the high tuition fees. I don't see an increase in this economic class. Furthermore, I don't see an increase in elite schools that cater to this group. In most major cities, there is only a handful of these elite schools. The tuition is certainly not decreasing either and salaries are tied to the tuition fees. So to answer your question, yes there will always be a market for high quality teachers at these upper tier schools.

So it depends on what type of teacher you are. If you are a new/inexperienced teacher, it will take you awhile to make your way up the ladder. During that time, you'll be working at low paying jobs. However, if you are a veteran teacher who excels at their job, there will always be a market for you that pays an excellent salary.

It really depends on the school. If not counting before school, recess, lunch or after school "prep" then about 75-90min per day. In that time, are collaborative meetings, etc so not completely individual prep. This prep time comes when those kids are in specials (Art, Music, PE, Languages).

Africa is a big place.

Morrocco, South Africa, Mauritius, Namibia, and Botswana are all great, safe places to live that I would recommend with kids.

To the OP: Having lived in South Africa with 2 young kids, I would completely recommend it. It really is a hidden gem and our favorite posting so far. Despite what the news might say, living here as an expat is great. It is safe, clean, beautiful, outstanding food/drink, very affordable and tons of family stuff to do. It really is nothing like the rest of Africa. Many of the restaurants have spacious outdoor playgrounds with child-minders. There are lots of preschools around as well. Unlike many other countries in Africa, it is not a hardship post.

Regarding racism, South Africa has come along way in ~20 years. It's not perfect and of course it exists but honestly, it seems much worse in the US.

Edit: Almost forgot, the healthcare is worldclass.