I think experimenting with this sort of scene can be quite useful. For example, you were trying to get the front two figures in focus, and you did that ok, which is good.
Is it as sharp as it could be? That depends on how you did the capture. 0.8 sec is very slow, so presumably you used a tripod (did you?). If you used a tripod, did you use a remote shutter release or the camera's self timer? Doing either of these means there is less vibration while the photo is being captured. And vibration makes images less sharp. Did you lock the mirror up? If not, "mirror slap" can cause vibration. Does your camera have electronic first curtain shutter? (I see it referred to as Electronic Front-Curtain Shutter in
this Nikon document, which you might find it helpful to read.)
Incidentally, having dust in that image was quite good I think when trying to assess sharpness. If it wasn't for the dust there wouldn't be any fine detail to examine in that image. You may do better to practice on subjects/scenes with more detail in them.
Then there is the issue of Depth of Field (DOF) - how much of the image is in focus front to back. You might want to experiment with using different apertures and seeing how that affects the DOF. That scene shows where the DOF falls very nicely. You can see that it is quite narrow. Getting enough DOF can be quite a problem when photographing small things.
When starting out you may find it useful not to worry about "artistic" issues and concentrate on getting a feel for some of the technical issues. The relationship between aperture, shutter speed and ISO is one of the major issues, and lighting is another. (You may have to use artificial lighting (probably flash) if you get further into the business of photographing small things. Partly it depends on your subjects. Your camera with that lens should work nicely for closeups of flowers (I use a somewhat similar Canon camera and lens for flowers), and I always use natural light for flowers. But if you want to get into insects etc smaller than butterflies then you probably will need to use flash and also get some additional glass. This might be a specialised macro lens or a less expensive addon lens (I use four different addon lenses of different power). As you are starting out with all this you may find it helpful to have a look at
this thread to read about the options for lenses. To see the many different ways you might tackle artificial lighting,
this thread may be helpful.
Oh, and if you like the look of mixed colour and black and white, go ahead. As with some other approaches though (such as "HDR"), be prepared for some flack. But ultimately it's what
you like that matters as far as your photos go. IMO.