I have the late 2018 12.9" iPad Pro (USB-C) and have supposedly one of the best SSD's (Samsung 500gb T5) to attach to the iPad, It pops up quickly in the Files folder on the iPad and works extremely well. However if I have a folder on the SSD with maybe 1,000 photos (most are 24mb RAW files) in that folder it will NOT open on the iPad. It tries and eventually just stops trying and will not open the folder. I also tried moving (copying) the entire unopened folder to the iPad with no luck.
I imagine if I modified my photo folders to smaller quantities of photos in each it might work but my MaBook Pro has no such difficulty opening and displaying the photos even if there are 2,000 - 3,000 photos in a folder. This is a serious impediment to any future plans I might have in substituting an iPad Pro for a MacBook Pro.