No flight logs are showing up when I connect to dji assistant. The drone connects because it checks the firmware, but the Upload Data section only 'keeps loading'
As you said, you connect to check firmware, so it should work with your cable.
If you exit DJI assistant can you see the uSD card in the Spark as a disk (assuming you use a windows PC) and browse the files on it.
Maybe you can check with this VIDEO if you follow the same steps to retrieve the DAT file.