@Ricken007: Sadly it isn't possible to set the SD card as your default storage location. What I'd suggest is periodically moving your personal media files to the SD card by heading to My Files > Images (as an example) > Tap the 3 dots in the top right > Edit > Select the files you want to move > Select 'Move' at the bottom of the screen > Tap the back button > SD Card > Select a folder, or create a new one > Move Here.
To complete @AndrewL suggestion, some apps manage to have the SD card as the storage default location. Some examples: Samsung Internet, Camera, Google Photos. There should be others. To setup this new location, you have to go in Settings of every app or the go in the Apps category for the global Settings. An example: Google Photos=>Settings > Apps > Photos > Storage > Change.
Hope it helps!