iMac boots up but no display after replacing HDD with SSD

My iMac 4K Retina Display will NOT power on at all: No backlight, no LCD (as far as I can tell, with a flashlight shined on it). It will boot up to an external display.

The internal display doesn’t even appear in System Report when looking at the list of displays.

I was extremely careful removing the LVDS cable. Thinking that I may have damaged it, I ordered a brand new one, which hasn’t helped at all. Of course I’ve also tried an SMC reset, NVRAM reset.

ALL I did in the original repair was:

1. Remove original HDD
2. Install new SSD
3. I was careful to make sure the iMac wasn't plugged to power when disconnecting and reconnecting the internal display

The fan also runs at full speed when the machine is powered on, which I suspect is just because it can’t see the internal display.

Apple’s diagnostics shows two issues:

There may be an issue with the System Management Controller (SMC).

Reference Code: PFM006

And:

There may be an issue with the display.

Reference Code: VFD001

I've also peeked inside when powering on the unit with the internal display connected, and LED1+LED2+LED3 light up. LED4 lights up but is faint. LED5 does NOT light up at all.

Any idea on what I could try out next?

@danj I've attached the pictures of the logic board + new eDP cable