Reply by Bill White

How long have you had the slab, and have you allowed it to equilibrate in the shop environment?
Big slabs such as yours will require some time to “settle”. After that, they will need to be hand planed. Even then they will wiggle more.
You’re in for some time and effort.
If you’ll PM me, I can give you a name of an excellent woodworker who may be able to help with your challenge.
No guarantees for sure, but he might be able to offer some wisdom.
Bill

-- [email protected]