Two millimetre variations isn't much - you'd get more than that on a sawn board of that width, so use the same technique you'd use to prepare such a board.
Personally, I'd use a jackplane with a fairly rank set and camber to the iron, and plane ACROSS the grain to remove the worst of the high spots. Work from both edges towards the centre to avoid breakout at the far edge. Check frequently with straightedge and winding sticks (two pieces of something straight and a bit longer than the tabletop is wide). Once it's flat, take out the jack-plane ripple marks with a fine-set try plane used with the grain. If you get tearout, use a finer set, close up the mouth, sharpen the iron and set the cap-iron as close to the edge of the iron as you can get it. Follow up with a smoothing plane set up the ame way. Do the back first for practice, then the show side. Finish with scraper and sanding if it needs it.
The trick is to remove the bulk of the waste with the jack plane, so the finer-set planes only have to refine the surface, not create it.
That procedure will be quicker than trying to set up a router in a frame, and a lot less noisy and dusty!
