Hello Amy
My experience of retention bonuses is that you are just delaying the inevitable – if people are going to leave they will. If someone wants them enough then they offer a golden handshake so the employee is equal.
The culture does need addressing. Cape Town is cool so your culture needs to match it. The only thing that worked for us as a tech company (raided by Microsoft which was painful) was to be seen to be cool and agile, we created a great working space and really focused on activities/projects/bigging-up the moment.