My experience: I use cream all the time, hot, cold, wet, dry rides. Great if you don't, but if you do then what I have learned is that some creams work better in hot than cool or cold weather. This time of year when it is 90-100 degrees and my rides are in the 3-5 hour range, then I use bag balm or another vaseline-like cream that can hold up to the heat. For rides over 4 hours I bring along packets of Doc's (mentioned above) and reapply. It seems you have done all the usual problem solving and maybe the hot weather and longer rides mean you might benefit from a thicker chamois cream. I recently developed a small sore/blister after a 4 hour ride on 90+ weather and it was, I believe, the result of a slight misalignment of my saddle.

I am applying a bit more bag balm and reapplying cream on longer rides as a solution to a similar issue.