Very good idea! A raised mound of soil keeps the root zone above the water table. And as long as you only build UP, that's all you need. I think you might be smart to keep it all above ground level. The drainage is good up there!
But if you dig DOWN into soil that doesn't drain faster than it rains, you just create a mud bath, which may be good for facials, but not for plant roots!
I made some raised beds that not only went UP, but allowed roots to penetrate DOWN below grade into partially-amended clay ... which at first meant "underwater".
I had arranged for the low points in some beds on a slope to drain down to a nice, low point.
I had remembered that water would flow downhill, but I had forgotten that once it got there, it would stay there. So now I trench so that the floor of a bed always has somewhere lower to drain down TO ... then I also build up a mound of good soil as you do.
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