I assume there's a world-building reason for The Sand Kingdom, because I have to say the contrast between the desert colour and the lush green is quite jarring! You don't normally expect those two to be right next to each other like that. I like what you've done with the sand in the bottom right of the map, though until you fill out your kingdom a little more I'm not sure what's the best way forward for you. I assume you're going to have cities, or are the people nomads or what? How are they feeding themselves and where is their water coming from?
As regards mountain brushes, just remember mountains come in ranges (unless there's an in-world reason). You can get away with one or two solo mountains but, generally speaking, you want them to form a line of peaks rather than have them dotted here and there. So, for instance, you might try a line of mountains running down the middle of island, perhaps with some foothills at the base, and you could dot some forests around there too. This might help everything blend together.
Look forward to seeing what you do next.