The thing is, your yard should fall (slope) from all directions to the gulley grids in the centre. If the grids are higher than the surround it would possibly require the whole surface to be raised. Any fixed plant or equipment would then be left in shallow "pockets"/puddles.
I cant make out what the surface material is, can you name it?
Depending on the surface there are some SLC's - eg. 2-part from Cementone/Bostick - that might do the trick. Do vehicles use the yard?
Perhaps, if you clean off the surface and put a straight edge across it, - from perimeter to grids - with a level on top, you might have a better idea of how its falling, and what dips you have to contend with?
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