Harmonic distortion in sub threshold 2 stage opamp
Well, using your much improved FFT you could approximate THD to the amplitude of the 2nd harmonic - it appears to be about -42 dBV and your main signal is at -15 dBV. The 2nd harmonic is therefore 27 dB lower.

As a ratio that is 22.4 times lower or about 4.5 % THD.

If you included the 3rd harmonic at about -58 dBV you have to convert to a real voltage and take the square root of the sum of squares hence, -42 dBV becomes 7.94 mV RMS and -58 dBV becomes 1.26 mV RMS. Now square and add them then take the square root to get 8.04 mV RMS.

Convert this back to dBV to get -41.9 dBV i.e. virtually no difference between using the 2nd harmonic on its own (-42 dBV) but, just for the record the difference is 26.9 dB or a ratio of 22.1 or about 4.52% THD.

In conclusion, unless you have a compelling reason to go any higher I would just approximate THD to the ratio of the 2nd harmonic to the fundamental.

Regards "stability analysis" I'm not sure what you require to demonstrate.