I've been using my Samsung SM-T550 for a couple of years. I have been using the QuickPic application to copy and edit pictures taken from the camera and downloaded then MOVE the pictures to the 64Gb SD Card. Have also used the File Manager app to do periodic maintenance and copy/move/delete pictures from the Internal storage to the SD Card. When I installed the Screen Master app I was able to set the default directory saved to the SD Card.
I'm guessing after the last Android update I no longer have any access from these apps to copy/move/delete pictures to the SD card? I can connect the tablet to my computer and do maintenace that way, or remove the SD card and place directly in the computer for the same. When trying to copy or move pictures to the card I get the error "You must perform the certain steps to grant SD card access rights". An example comes up with 3 pictures showing how to do this, but none of those options are available?
Have you tried downloading another app that may be capable of this?