I paint with acrylics as well; mostly Reaper, but I started with Andrea and then some Vallejo. You will find that the mechanics of flat acrylic painting are the same as for a round (3D) - brush control, layering, wet blending (I see you use a wet palette), base coats, glazes, and washes.
As Dave pointed out, light is what you control. Pick a direction from where you want the light and paint accordingly, but be aware that some parts of the piece might be casting shadows on other parts. It is the interplay of highlight and shadow that gives the piece a 3D look, so you might find yourself moving towards increasing the light/dark contrasts.
What you have so far looks good.
Press on!
Cheers,
Glen