"filling the cracks" what cracks would that be? There are various floor patch products.
I am not a big fan of SLC over planks. They expand and shrink cracking SLC. Even with felt or other barrier thin sections may still crack. You need to cover entire floor with 5/8 or 3/4 tongue and groove plywood. Level that ply as much as you can with the graduated strips on 16" centers..
You know what caused sag, what is the cause and have you corrected the problem? Is the floor springy or bouncy at all?
The under pad should be whatever the maker of flooring you choose recommends, over what ever your final sub-floor surface turns out to be.it won't be cork.
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[CENTER][FONT=Comic Sans MS]If you hook your thumb over your belt you won't hit it with the hammer or leave it layin on the saw table.[/FONT][/CENTER]