Just a couple of points from my point of view, though may not be relevant. All advice says the substrate should be higher at the back and gradually lowering towards the front, making it in a slope like fashion, which makes the debris sit down at the front of the tank, easier for cleaning.
Aquascaping advices I have seen talk about a triangle shape or U shape for design. so either left to right or right to left, highest at one side, going lower to the other
side. This
site has some nice ideas and pointers, I plan to look at it as well
Vallisneria is a prissy kind of plant. Needs high level of substrate, doesnt grow in the direction you want it and at some point, the roots grow through the substrate fully, I tried to get rid of it once and realized I didnt have substrate, I had just root system with bit of sand and it looked like this and I started with like 6 plants and this was a year after
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Corydoras appreciate a shade and resting place UNDER things, so if you get more wood and rotate it to create a cave sort of system underneath it, they will appreciate it more. You wont, cause you cant clean under it but hey, that is life.
So my overall advice is to go more. More substrate in the places where you want rooting plants, more wood, more stones that need to be near each other. Lot of open spaces for the corydoras, more stones in a sort of hill setup,...