Don't sand to try to achieve flatness. Think of all that time, all that paper, and all that dust...... =
Cutting the fibers leaves a better surface than sanding, which shreds. = Handplaning

Unless you are limited physically, maybe then its the only way.

First with a #6, then a #4 followed by a scraper = much better finish.

And don't try to achieve perfection in the glue up = forget the biscuits, the dowels, the dominoes, whatever.
Glue it up thick and hand plane it down to perfection.

The only sanding you should need to do is final touch up on edges.