I’ve spent time in Joburg & Durban and found them worthwhile visiting. Cape Town is certainly the jewel in the crown, I’ve tried to swim in the sea around there but it is simply too cold, except for a small spot around Simonstown. There are beaches north of Durban that are swimmable with warm water eg Thompson’s Bay in Ballito. I’ve been to the Seychelles and wasn’t keen on it at all (only visited Praslin though) check the weather for the time of year as the rainy season is not a pleasant time to go. My daughter went to MRU and didn’t enjoy it so that’s put me off going.
Kenyan beaches are quite nice, we stayed for a week somewhere north of Mombasa, water was very warm.

Mozambique would be a fabulous add on for a beach trip, many options depending on how far you want to travel.
there are also many great options in Durban , it’s so beautiful if you go up North!