Well, alrighty!
Somewhere on the spectrum between allowable wiggle room and cheating, I've caught up. Truth be told, I fell egregiously behind this summer and when I thought about having to read four books from my challenge to get caught up this month, my heart sank. But then I had my lightbulb moment:
I'd doubled/tripled up on books in the early months of the year and had in total read eight books from my StoryGraph Challenge; it's just that they were concentrated in five months. A little creative/judicious rearranging (keeping essentially to the themes for the months) and bingo! I am now on track.
If not quite to the point of the challenge, I'm content as to its greater purpose; i.e., I'm now likelier to finish it.