I have been thinking a lot about what's missing in your value studies, because overall the values are very good and accurate. What's caught my eye are 2 things:
Gradients: I don't mind this as much, because it's just part of the simplification process. In some cases it would add a lot of depth though. If you look at the first study (first image) you can see a very subtle shift from dark at the bottom to light at the top, whereas your study looks more layered. It's especially noticeable on the reference where you can see far into the landscape and quite consistent in your studies.
Edges: This one bothers me a bit more, because a huge part of why value is important is because it helps the read of an image. I think it might be beneficial to take a little extra time to make sure the edges of your objects are clear, because some of the edges get lost in your studies. All in all your edges are a bit too soft compared to the reference.
I tried to implement what I recommended but the changes are very subtle. It gets more noticeable when you overlay them on top of another and switch a layer on and off.
Not sure if that helps, but I wanted to offer what I noticed. All in all, great job.
