I received some advice that emerged the same issues as you.

I think I'll work on it and edit it by then. Thank you for this valuable feedback ! :)

Really nice and all, but you've extended the black on the bottom far too much, making it unneccesary and the clouds can use AA or dither, because they look too.. "blocky". Good job, though! Needs a bit more highlights on the clouds, but other than that, it's really good. I just noticed the  Are those buildings or mountains? It's very vague.