I would suspect it will be a lot smaller than 9gb if your prune it. My library has something like 1.2k movies, 330 TV shows, and 2.5k albums and last time I dumped my thumbs and had the texture cache maintance utility force a rebuild it was only somewhere around 3gb and that was with my art set to cache at a larger resolution than stock OSMC.
Don’t get me wrong, doing a path sub and storing your cache elsewhere is totally fine. I outlined how to do it in my path substitution how-to on this site and I have tested it thoroughly and not found any issues. It even seemed to be slightly faster loading the artwork with the art stored on a SSD on a network attached PC in some cases. It is just that doing so for the sole reason to keep tens of thousands of small art file that will literally never get accessed, especially if these are also getting backed up, seems like the wrong approach.
I wonder if its being maxed out when my father views photos on the TV?
His JPGs ranging anywhere from 7.5mb - 15mb and the RAWs between 35mb - 45mb…
Not as far as the cache goes. If memory serves Kodi by default doesn’t cache the photos at all and if you did cache them the size of the originals wouldn’t factor in as the thumbnails are quite small. The main reason I think most people get stale images built up is by either by rescraping sources or by using using add-ons that display an ever updating list of different artwork. What I mean by this is that if you have an add-on like Plex then most art is static and only new thumbs get created when you add new content to your plex server. On the other hand if you use something like the Youtube add-on then your probably looking at a lot of new images every time it refreshes your home page and after you see them a few times you frequently will not see them again, thus buildup of bloat in the artwork cache with no benefit.