The flex cable issue was more of an issue in the 2016/2017 13" TouchBar and Fn displays.
The 2019 had an issue within the T-CON unit with bad soldering, but I've only seen color bars with that issue not a backlight problem. Even still that doesn't explain why the external displays aren't working, did you try a second USB-C to HDMI unit? as it could be thats the issue with the External.
In any case the backlight logic on the main logic board will need repairs to fix the internal display, its possible there was a power issue between the outlets you used for the systems charger and the one the external display was using that zapped the logic. This is when the ground connection is hot or just not connected. I would get a simple cheap plug in outlet tester to see if the outlet is mis-wired or has a bad/missing ground.