black screen on boot, chimes, no capslock light, new board didn't fix

One morning, this laptop just didn’t work anymore. When powering on, it makes the startup chime and the screen gets slightly lighter (pretty sure it’s the backlight- the apple logo lights up), but that’s it. The screen stays black. The capslock key does not light up when pressed.

Here’s what I’ve tried so far:

1. reset NVRAM [1]
2. reset SMC [2]
3. tried with different power adapters plugged in, including no adapter (battery only)
4. tried without the battery plugged in, running only on the AC adapter
5. tried with an external monitor [3]
6. replaced the entire logic board (but not the DC-IN board)
7. tried with and without the m.2 drive plugged in
8. tried removing the wifi card

[1] This makes the screen backlight “zap” off and restart the system as expected. AFAICT it resets properly.

[2] After performing the ritual, when powering the system back on, the charging light on the AC adapter momentarily goes green, and then back to orange. AFAICT this is the expected behavior, indicating “success”.

[3] The external monitor recognizes a device got plugged in, but fairly quickly determines there’s actually no signal and goes to sleep.

At this point the only thing I can think that may be the problem is the screen and/or the cable. However since the backlight appears to work, I’m not sure how likely that really is.

Any thoughts / better ideas?