I'm in the same boat as you. Coming from PC/UNIX/Android world I wanted to just copy my files onto the iPad filesystem and then view and edit from there. They don't make it that easy.
I copied my files to the iPad using iTunes Photo sync and that seems to work but I'm having trouble with my RAW files, they seem to get converted in the transfer although they show as RAW in iPhotos.
I did manage to copy a folder from my Google Drive using Files and I might try that for more photos, I wanted to avoid two 50Gb wireless transfers though.
I tried a 3rd party lightning to USB/SD adapter with no luck, It wouldn't power the USB stick and didn't see the SD card. I returned it and got an Apple brand USB adapter and it handles the SD card well.