I've got recently KC901V and playing with it, it's a very good and promising product, but it has some weak points.
I see, in S21 measurement it has poor isolation on 5-7GHz range, real measurement range is only 40-50dB. But if I need to add 3-10dB attenuators to improve ports 50om matching and remove measurements ripple, the useful range becomes even smaller, down to 30-40dB. It's even worse, than s21 measurement on some cheap products, like NWT6000 I have.
I think, most leakage comes from internal RF switch, and this leakage should not change phase depending on DUT load, so this leakage can be compensated by calibration.
I think, as you have a vector S21 measurement, you can record S21 vector response (with disconnected output and input ports), remember it in internal memory and subtract it from real S21 measurements. With this calibration, you will have 60-80 range on 5-7GHz, and this will be a great improvement.