I am no expert but from the little user experience I have with NextCloud, it is possible but not via NextCloud alone.
You may need to have a Linux script running on that pie to always use a cp command copy all any data out of USB plug in device to a specific folder of your NextCloud user data
NextCloud won’t detect those files as its still not in it’s database, so you may also need to include the command to rebuild file and directory structure for the database after that cp command has executed successfully
NextCloud isn’t designed for this exact role, rather a portable NAS or NAS related applications are. For your use case, may be that is a better choice.
NextCloud is designed to allow you use internet from your Trip’s location to backup the SD card on a nextcloud server running at home or cloud. You don’t need to carry a NextCloud with you or wait for trip to end and return home to plug in that SD card.