(Popping up after years of absence.)

Andrew, I flatten my stock by hand, but this looks pretty extreme to me.

Christopher Schwarz seems to think it is easy to flatten a workbench as a
newbie, but that was not my experience.  I tried to flatten a workbench
that was pretty flat to begin with as my first exercise in flattening and I
ended up removing too much material in the wrong place and significantly
tapered the thickness of the top.  I was under the delusion that if the
plane is cutting, it must be removing a high spot.

If you are going to flatten by hand, don't start with this board.  Start
with a section of 2x6 and then try a section of 2x12 from the Borg. Yellow
pine is great for that if it is available.  Use something soft without
knots.

It's easier to start with the concave side up.  A long plane will ride over
a hill, cutting the entire way.  However, a long plane won't cut a valley.
So if the concave side is up, you can use that to your advantage to remove
as little wood as possible.

Ripping it in half will help a lot.  It would also let you make sure the
pith is removed and will preserve more overall thickness.

I have not used an adze or hatchet, so I can't comment on that, but Roy
Underhill seems to do that mostly with green wood.  This slab looks pretty
dry.

In the olden days you would get wood from a sawmill powered by water
because reasonable people would not make boards by hand.  In this case it
seems reasonable to apply the same concept, especially since you need the
two sides parallel.  My local hardwood lumberyard will flatten slabs for a
very reasonable price.  If I were in your shoes, I would seriously consider
that.  To me this is the kind of exercise that newbies try by hand and then
conclude that hand tools are useless.  There's no reason to get discouraged
early on.

Mark