You can run these thru a planner or a wide belt sander but they will still have some shape to them. They will be smooth but they will not be Flat. If they are bowed or twisted they will still be bowed and twisted.
The CNC is the perfect tool for this process. I can take a piece as large as 50" X 101" and up to 6 " thick and surface the two sides and It will be perfectly flat and the same thickness throughout.