Yes 1605 is more than loads of time .. Airlink's schedule isn't the most frequent but they're generally reliable .. I find Safair also very reliable and have the best frequencies on this route ( sometimes flights 30 minutes or less apart ) with more options when things go wrong ( never happened to me ) .. Airlink may be (slightly) more pleasant, but Safair is fine and as I say super frequent and reliable .. I would avoid SAA and Cemair

Never tried thru checking on Safair, but I doubt they would being a low cost outfit .. Domestic baggage delivery is very fast and JNB usually and it's only 5 minutes walk to the International check in desks .. if you're coming in on the 1605 it's pretty likely you'll be at the BA checkin desks more than 3hrs before your 1945 departure !