The noise budgets below show noise from coating brownian, TO noise and TE in substrate. The three plots are from 52,54 and 56 Layer coatings.

 All the designs have 1/2 cap of nL, with nH ending on the substrate surface.  There are no significant differences in the noise level at low frequency, since TE noise in substrate starts to dominate. I used the substrate

parameters in thermal fluctuations, so the cut off frequency for TO calculation is low (~ 3 Hz instead of ~ 200 Hz). The design can go for 56 layers.

I'm thinking about another solution, where the top layer is nH, followed by 1/4 layers. If the first nH is 1/8 lambda thick, TO can be cancelled nicely (for 56Layer + nH cap). The transmission is 140 ppm , which is in the chosen range (100-200ppm). But I feel that the 1/8 cap is not good for a high reflectivity mirror, since the phase of the reflected light within  that layer is not really inphase or out of face with the light reflected at the air surface. I'll think about it more to see if it would be a good solution or not.