If possible, you need to make it possible for 3-5h of transit time in Johannesburg, so you have enough time and energy to capture your connecting flight. Transiting in Johannesburg is quite easy; merely follow the indicators for ‘International Transit’. The flight from Johannesburg to Harare usually takes about 1h https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/