Good news is that the Mac and graphic card are still working. Bad news is without parts at hand or logic board diagnostics abilities you won’t be able to do any further troubleshooting. I wouldn’t recommend anyone to buy such an expensive new screen assembly to “see if it works”. It might be the screen as well as a logic board issue. Try reseating the screen connector on the logic board, but if that doesn’t help you’ll need to find a technically reputable lab into component level repairs.