Improving render-skillz

First thing I strongly advise is: stop shading with black (or any super dark color). Shade by changing the hues of the color manually.
Second, use references, and here's where it get harder - you need to really study them, not just the 'what' is on them, but especially the 'how it looks'. Like at the second picture, you saw a beard, your mind did "ok, now we draw a beard" and you painted what you think a beard looks like.
What you needed to do was to look at how the hairs of the beard grow and interact, where the shadows are, how the light changes the hue, what different colors are mingling there. To make a study, you literally need to study the picture.
I hope I said it in a way it makes some sense

AND the best trick that made me fall in love with rendering - make your studies greyscale. Make your reference pictures greyscale and study off of them. It is infinitely easier and once you get a good hang of it, adding color will be just one more step.

PS: after you put down a color, it's not "cheating" to colorpick from reference to see if your color is off or not ( + try to think how and why). After you finished a picture, you can put your reference on top of it to easily see where your picture is most off. Learn from your mistakes!

Rendering all comes down to understanding shape, form, and value. Can't go too much into now, but more than willing to explain any of the three if you'd like!

ye your using some super dark tones in places they don't belong. would recommend trying to use a fairly soft brush and then go in with a harder brush if needed. might be good to even colour pick from the original painting to see what colours they have used and just get a feel for their colours.