Where is the space available on my SD?

[Nachteule]

df output looks ok (/media has 1.1T so I suppose there was an external disk attached), so I see two possible reasons:

1. you have still one or more snapshots which consumes all the space
-> delete all subvolumes expect */@ (at least you did not delete the last_good_known snaps)
2. your FS is corrupt
-> fastest possible solution: make image from your current sys and reflash card again (see here)