SDR Signal Generator Frontend
I'd split your project in two parts:


the power metering part
the adjustable attenuation part


If you want to correct how much power you're actually producing, there's no way to know but measuring. So, you're on-spot with the split-and-measure approach you're taking here.

There's interesting power meter ICs that you can buy that are pretty impressive in specs, so maybe that'll be a good way to go if your own measurement equipment is limited. Analog Devices is the main supplier of these.

Your fine-adjustment hardware, however, ignores one key fact: you're using software defined radio. Rather than using hard-to-calibrate analog attenuators after your power metering, where you can't tell whether they behave correctly, just use the power of SDR!

Just scale the digital numbers you send to your SDR frontend. Depending on the application, the baseband signal processing is often even done in floating point arithmetics, making such adjustments a breeze and low-noise to implement. 

You then would've built a TX-side AGC, if you logically connect the measurement to your amplitude scaler. That saves you all the analog attenuation troubles.