South Africa issues critical skills work visas that last for 5 years.
From my knowledge, there is no six-month option or any other options that allow for a period shorter than five years.
The advantage of the critical skills visa is that you can live in SA and look for a job for up to 12 months.
I recommend contacting an immigration company. They have expert knowledge on this kind of thing. Otherwise, this Expat Arrival's page on work permits in South Africa might be useful.
Hope that this helps.