Out of curiosity -- Which pin is not connected in that 15-pin VGA connector with 11-pin cable if you compare that cable with that 12-pin cable ?

Anyway what about trying? While the PCB board headers are not direct match
the cable is STILL usable after modification -- just you have to wire the pins for R G B signals and their returns, and Hor/ver sync.
Even if you just connect these 5 pins and one ground you get the picture already. The rest is just to connect the remaining grounds too.

in feebay when ordering from malaisia the Price can be 2GBP for a 1.5meter VGA cable (which you adapt by chopping off the other connector) as you already know how to open the monitor then the rest is easy.
Also option would be to lurk around the second hand shops and you see how many audio cables, power adapters AND the VGA and computer cables are there as well for perhaps 1..2GBP per piece. And then you use that one instead.

As a last resort, just get some 75ohm thin coaxial cable, or atleast some shielded audio cable and make yourself that cable to connect these 5 pins and their grounds. I did in the past with shielded audio cable only and this worked up to some VGA resolution with 1..1.5meter length cable great.

Do you understand? with any of these options - get soldering iron hot and multimeter handy and solder together those 5 signals and 1 ground.

I do appreciate these tips for sure, but the issue is that I suck ass at soldering. Most of the times I've tried, I end up making things worse. I do have a multimeter, but have no clue where it is. I'll have to find it sometime and perhaps I'll attempt one of the options you've given, but I fear I'll just end up completely breaking something. But you're also saying that there's really no spare stock of just these internal cables anywhere? That's kinda sad.

If the internal connectors are the same style, you should be able to use a small screwdriver to release the pins and swap connectors. No soldering required. Make sure to match the pinout to the HD15 connector.