300dpi is usually acceptable resolution for a document - depending on how fine the print is. 200dpi is passable. SO a 31-inch doc should be around 6200 pixels wide. That’s a bit more than a 24Mp camera can produce. Then there’s the issue of focus, etc. That’s why I suggest at minimum stitching two halves with a better (24Mp) camera.
The other issue is flattening, based on the picture. At very least, a series of magnets along the edge, best is a sheet of glare-free glass (if glass and a polarized filter don’t work)
At a certain point, this gets to be too much work if there’s a scanning service nearby. Depends again what quality and resolution is acceptable.