Custom thumbnail when viewing home videos by folder

I have a ton (7TB) of training videos that i’m trying to add to plex. From my searches it appears that the only decent way to display them in a logical (to me at least) structure is to use the home video type and view by folder.

So i have for example;

Educational (root level)
— Psychology (i would love to make a custom thumbnail for this)
— --- Cognitive Behavior Therapy Retraining your Brain (again, a custom thumbnail here would be very useful)
— --- — 21 video files (these i can set thumbnails and descriptions, etc already no problem)

Any suggestions on how to accomplish this? if need be, i am database proficient.

Thanks so much for your time!!!

is this not possible? would really be helpful to be able to set custom thumbnails for folders for sections that don’t currently have a way to match