The issue:-

Device permanently stuck on a black screen with backlight and no image, or the device would sit on a black screen with no image for a long time before displaying the Apple Logo and loading the OS.

Problems found during diagnosis:-

R8153 and test point VMON_Q2_BASE corroded. R8156 and test point S0PGD_C corroded. C1910 severely corroded and the attached ground plane at pin 2 of C1910 has corrosion under the PCB mask.

The fix:

Replaced R8153 and cleared corrosion from test point VMON_Q2_BASE by soldering over the test point. Replaced R8156 and cleared corrosion from test point S0PGC_C by soldering over the test point. Replaced C1910 and missing PPVRTC pad (pin 1), peeled PCB mask off the ground plane at pin 2 of C1910 and cleaned the area with a fiberglass pen, then covered the area with curable PBC mask.

Issues resolved, device back to booting properly and it no longer sits on a black screen.