Pout is relative low, why that big attenuator then? 20dB attenuation => 20 dB reduced dynamic range of VNA. How much power can your VNA handle?
Measure TX impedance requires that the chip can be set in CW mode, I assume?
Measuring VSWR of antenna do not result in impedance values? What type of VNA do you use?
Measured TX out power level can be improved, in optimal case by 2 dB, which probably not will give the user any noticeable extended range.
Do you plan to improve RX in similar way?
If the circuit and PCB is designed in a acceptable way from RF view, but improvements in coverage still is needed, had I started with measuring the antenna efficiency. Improvements of the antenna is often the were most amount of dB's can be found. Improves both TX and RX.