Firstly, gotta say I love this mock-up and I can't wait to see the finished game. Always been a big fan of you art. Moving on, I'd like to start off with an edit of your purple chest. Here's a comparison of your version (left) to my edit (right);
Going into detail about the edit, I made the lightest shade of purple less, for lack of a better word, obnoxious. It doesn't stand out so much from the other shades now. This was probably my biggest gripe with the original. Going on ahead, I used a base as an example to keep the shading consistent overall. I've included a step by step below showing this process;
Finally, I've done another version of my edit. The left one being the original and the right showing off a solid line of colour where the purple meets the gold on the top half of the chest.
It's a veeeery slight change and barely even noticeable in 1x but I felt it was worth mentioning. In any case, hopefully you'll find it helpful, I know editing your version definitely helped me learn a bit.