I don't spin down the hard drives so I don't really have that problem.
At first I was thinking in setting thumbnails and database in SSD cache and photos on array. This was because in photoprism requirements it says:
"Indexing large photo and video collections significantly benefits from fast, local SSD storage and enough memory for caching."
But maybe you also need the photos in the SSD to really benefit...
Also I did not considered the thumbnail space, tomorrow I will check to see how much thumbnail space is required for X photos.
I think it will be better to have both options (maybe add a disclaimer that to really use the storage path, then the user should add the variable PHOTOPRISM_STORAGE_PATH). I myself, always prefer to have the "media" locations and the "configuration" locations separated.