Ideally your test strip with f/stop printing will determine your highlights, then you alter your shadow rendition by changing contrast.
It sounds like you had a very strong contrast situation, and that you have overdeveloped the highilghts. You are on the right track with using compensating development (5min agitation), but your development time for this neg may have to be shorter.
Instead of using very soft filters, which tend to produce low local contrast in your print, I recommend practicing flashing, which helps preserve local contrast better than using lower contrast filtration.
The idea of burning in the sky with Grade 5 filtration is also a good one, because it does add some texture to the print, if there is tonal variation in the highlights of the negative.