I expect several are interested in the possibility of more acceptable results with your relatively low cost lens/converter combination.I will try and get some photos with better light behind me rather than behind the subject. With luck it might be possible to get acceptable results with this set up.
The nearest Nikon/Canon/Sony option I am aware of is the Nikon F mount 800 f5.6 with 1.2x calibrated to the lens converter.
At at least ten times the cost of the combination you are using - few can afford it.