A week ago another supporter said exchanging the entire panel for a new retina one would be 350 € or less.
This sounds totally off. Does anyone here have experience with MacBook repairs?
They generally have a tiered repair pricing model and I think minor issues like the flex cable would cost around the cheaper price you've been quoted for and replacing the whole display would be the higher price.
https://logi.wiki/index.php/Apple_MacBook_Flat_Rate_Repair_P...
Follow the process there, which depending on your system could be as easy as remove the bezel, undo 4 screws, lean the screen forward, snug up the connector & put a fresh piece of kapton tape over it to hold it down and then put everything back together.