I'm back to the topic of THD measurements. I've upgraded Clio Pocket to version 3, it turns out that there is no need to replace the USB module. In fact, it would not even be necessary to perform new measurements to get THD graph, it would be enough to open the .crp file in the new software and select "THD" instead of "Frequecy response" in the options.
I'm still a little unsure with these THD measurements. The peak above 2kHz is ~37dB lower than the fundamental signal, which translates to 1.4% if I calculate it correct. However, previously (using a series of FFT analyses), a similar level of distortion was obtained for 96dB and now for 86dB. I wonder if everything is OK with my measurements. Anyway, THD 1.4% is not a disaster, but compared to available measurements of similar systems, the result is not impressive.
Mechano23 THD: gated (green), not gated (pink), fundamental gated (red)