I like the design and the mood but isn't this kinda huge-huge?
I don't mean to say that huge-huge is wrong, but exactly how do you plan to use your pixel art skills in a huge piece? It just becomes an excercise to do manual aa that will look the same as auto-aa if you're good, and of dithering to mix shades that will look the same as a cgi painting if you're good. I'd shrink this like, by four and work on it on the pixel level. If you can't see the pixels, what's the point of it being pixel art? If you can't appreciate it on 1x zoom and then go to 2x or 3x and look how harmoniously the little pixels are making up an illusion of something infinitely more detailed than really is there, what's the point?
But you say you don't want to reduce, so alright, here's some critique on the stage where you are:
you're shading every surface equally. You need to 'carve' volumes with your shadows. Here's a dirty edit with the darken brush in pro-motion. I DON'T SUGGEST YOU SHADE LIKE THIS, just to show the darkness is your ally:
