If the camera doesn't let you adjust the ISO level in-camera & you can't reduce the lighting to compensate, I'd suggest the simplest fix would be to get a new camera.

As you note, once you have the image, there are ways of darkening it, but, as you also note, that's not really the same thing.

I understand that's not the answer you want to hear, but it's the truth.

The camera turns light into pixels.

If those pixels are damaged--such as being under or overexposed--there's not much you can do to those pixels to get them back to what they're supposed to be.

If your camera is overexposing your image, the options really are to either change the camera settings so it doesn't do that anymore or reduce the amount of lighting to match what the camera is apparently expecting.

If neither of those are feasible, you're really only left with the option of getting a different camera that either allows you to change those settings or is hardcoded to expect the proper amount of ambient light.