If your mind is set on changing, why not look at a model that will not depreciate as fast?
For around half the cost of your G20 quotation (around R400k), you can get a 2016 F30 330d, which is significantly better than the 320d you have, yet affordable, and which will not depreciate as fast. Then, if you can afford the installments, R800k will get you into a in a slightly used 2017 RS3 sedan, which I guarantee will not depreciate as the 320d will. Keeping it in house, I saw a low mileage ( under 30k) F30 M3 competition sport the other day for R780k, which will also depreciate less than the 320d. Except for the fuel savings, both cars will perform and drive way better than the 320d. Yep, maybe not the greatest choice as daily drivers (but why not!!). There are several good BMW X3/4/5/6 models up to that mark, with at least 3 years of plan left, which can serve as daily drivers. Ultimately, there are so many better choices at the R800k mark - driving one of these would make the G20 owners jealous when they drive past you!