I haven't read everything hear but I get the gist. It's the photographer and not the camera who takes the photos. The camera you have is perfectly capable of doing a lot of good things but you just need to learn to use it to it's maximum potential. In fact you need to learn to use your mind to the maximum potential as that's what'll give you the results. If you're going to do anything I'd buy a new lens over a new body but! and a BIG but it is! only buy that lens if it's to do something new and NOT because you think it'll improve your photos. For example I mean if you really want to do landscapes but you feel 18mm isn't wide enough then get a 10-22mm providing you'll use that lens a lot and wisely and not just for the sake of it or for the fact you think it'll make you a pro when you put it on your camera.
Or instead of spending money why not find new and creative ways to use the gear you have? I watched a video the other week about landscapes because I wanted a new lens. This guy used a 50mm prime lens to do landscape photos and I was impressed with the results and the creativity that came from it.
Landscape photography with 50mm prime lens
Watch the following video and it'll show you what you can do with a camera that isn't near as high-spec'd as yours. The camera used is a Nikon D40 which was from about 10 years ago. The D5200 on paper trumps it in many things.
It's the photographer not the camera