It is possible to do what you are trying to do, and sometimes using a camera as a scanner bed does work very well. But on problem with cameras is that the closer you get to the edge of the lens the more distortion you’ll see.
Like, at one university I went to the large bed scanners were all cameras on tripods held over a brightly lit surface. Those were built from wood, but a metal tripod also works and can be more flexible.
The trick is to get the camera perfectly parallel to the surface being scanned, and to get enough even and consistent light on the image.
The stand can end up looking pretty home brew, but you can get really good results.
It isn’t always better than using a scanner, but there are situations where you actually can get significantly better results with a camera than a scanner.
Above all, making sure you are putting enough light on the image that you are taking a picture of, and making absolutely certain the camera is held in the proper alignment to the image is important. And the more you can avoid using the WHOLE imageable area on the camera the better you’ll be.
If you see too much distortion in the image even after doing all of this, then there’s a chance your lens is just not giving a flat enough result, and you might have to do some PhotoShop wizardry to get the image to be ‘flat’ in the final result.