Here's my trusty smootherator (my Stanley no. 7 plane), part way through making the top lovely and flat.It's a hell of a workout and leaves me exhausted but satisfied. There';s no better way to flatten a bench top. Just take lots of super-fine cuts in a crosshatch, at 45 degrees to the grain orientation.
The structure chosen allows us to easily build up the top without resorting to a gazillion biscuits and clamps. Instead it's just assembled on the cross-pieces, with PVA and nails.
Here's a view along the bench during flattening. A lovely, long accurately straight and flat work surface.
Yet to go is to put shelves in and build cupboard doors, but that can wait until after I've made my mast on it.