I don't think you can do 200ms. There are 12 divisions and your window seems to be 12X your timebase setting, unless there is some other way to change it. Still, 120ms works for 50 or 60Hz.
I only got decent results at these low frequencies using "Trace" and either the Blackman or Hanning windows. For those of you following along but without a DS1054Z, the noise at -60-70db is high, but not as obscuring as you might think. In real time, the noise is flickering while the harmonic humps hold pretty steady. You can easily see them and pick them out, even on "clean" power like mine. I think we can conclude that for all its faults, the FFT can readily distinguish between clean power (mine) and marginal or not clean power (spiff72).
This capture is my last, best attempt to illustrate and what we can see and measure. This is using a 4kHz LPF, although there wasn't much difference between that and the scopes own 20MHz LPF.