In my opinion this isnt the right application for pocket screws. Theyre more useful for small joints like 'rail to stile', or 'shelf to side panel'. Some people use them for edge joining but i dont think thats gonna help you here.
As mentioned above, boards on edge screwed underneath would probably work. There are a variety of brackets and fasteners u can use to allow for wood movement while still holding the top flat. I like the figure 8 fasteners personally.
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