In most of what I've seen in the topic, and the link you gave with some of your earlier work, it's just a lot of drawn lines and gradients. I've seen quite a bit of solid advice here, and I feel like you have ignored a lot of it. I'm no expert when it comes to art, but I you really should try starting with a smaller size and a simpler color palette, so you are getting more practice on the pixel level.
You claim that the challenge of working on a larger scale is worth it, but I think right now the larger scale is just causing you to use line tools and gradients to get a relatively cheap and simple looking art style. I quickly scaled down your image to 25% and recolored it a bit using just 8 colors. It doesn't look great, but it's a pretty rough edit, and it's clear that even with a much smaller scale, you don't really lose much of the detail you've included. I would recommend at least shrinking to 50% size for now so you can concentrate more on how it looks at the pixel scale (it is "pixel art" after all
)
I'm non expert on this, but it's just my opinion.