no video on screen

i picked up a 7+ (256GB, unlocked, no iCloud, "no power/doesn't turn on") really cheap to rehab. first thing i did was plug it in to charge overnight and immediately felt the buzz from the connection. connected it to 3uTools and get the full readout on the PC, but nothing on the screen. did the troubleshooting to see if it could be a blown backlight on the LCD but didn't see anything. put that screen onto another 7+ and the screen worked fine. put a known good LCD onto this phone, got the same result. close inspection (not with a scope, used my regular iphone and zoomed in) showed no issues with the video connecter on the motherboard.

at this point, i'm figuring it's a video issue on the motherboard and am looking at some services that specialize on motherboard video issues. but before i commit, are there any other troubleshooting steps i should perform? are there any other conditions/scenarios i should consider before going with my above conclusion?