Two weeks ago I've made few tests with my SDR RS2, connected on a SMG 0-1GHz R&S.
Unfortunately I've not saved the results, maybe I will proceed to another tests in the coming days.
However I remember that RSP2 is pretty good in level accuracy and can be used as a not expensive level metter.
I used SDRUno to get the results.
I totally agree with you, SDR programs are not really suitable as "Spectrum analyzer".
Fortunately, I own a Spectrum (HP8594), I'm a Lucky man.