CSC was my first thought and you did indeed have both sticks back, which is just a nudge away from both sticks back and in, ie the same as how you arm the motors is how you disarm them.
However my impression, from reading your description, is that you just flew it into the ground.
GPS mode means the craft will hover in place if you give no input, but if it loses GPS lock then it will float around but still at the same height (because is using a barometer rather than GPS).
Setting "Don't change mode" means you cannot accidentally switch out of GPS mode but it doesn't stop if from potentially losing signal.
Now if you were to give it full stick down it would descend quite slowly. This is to prevent it falling into its own turbulence and tumbling over - but you ALSO pulled it fully backwards, so the "Descend slowly" thing doesn't apply - you dive-bombed the pavement, basically.
Now, if your inputs were to rescue it because it was already in trouble you could blame the drone, however rescuing it would be full left stick UP to prevent it falling. You gave it full stick down, so I'm sad to say I think the trouble was you, panicking because it was drifting and giving error messages.
That's just my blunt and honest opinion. I hope for your sake I'm wrong and it was a machine error that DJI will put right, but if I were you I'd be looking at how to raise funds for repairs.