The gravel fram I built last year features both of your preferred solutions, a segmented rear AND dropped stays.
I think 20-25mm of drop will not cause any major issues as long as there is still some distance to the beginning of the butted section. Of course, depending on what you’d like to do with this frame… Seated 20ft huck-to-flats might not be the best idea 
I actually did some FEA to compare the different possible configurations of how the segmentation can be done and couldn’t find a significant difference in stress for any of them, so you should be fine on that one.
