How to paint layered complex backgrounds in pastel - "Dejected" WIP time lapse video

How do you tackle a pastel painting where the background is quite complicated and layered? Like a mess of foliage or branches? How do you retain your detailed initial sketch without making the background feel 'segmented' - as though you are just 'painting between the branches'?

Too often when I look at acrylic paintings, they appear flat or two dimensional because the background has been added as an afterthought (painted in around the subject), or the artist has used an airbrush to add the background in its entirety before they start work on the subject. Unfortunately, because of this lack of interaction between the subject and the background I tend to find that these paintings only have two discernable layers or focal planes - the background (which is usually totally out of focus), and the foreground which is usually portrayed in sharp and consistent detail throughout.

Pastel on the other hand is a beautiful medium for easily creating paintings with great depth through the use of multiple layers and blending. At any point during the painting process you can move the focal plane by blending layers and blurring outlines to send objects backwards, or by adding detail and sharpness to bring the subject forwards. This also lends itself to greater harmony within the painting, because you can constantly reassess the placement of layers throughout the painting process, ensuring that everything fits snugly together.

But how do you actually create a painting with a layered and complicated background? I decided to try out some new techniques to achieve this in a wildlife pastel painting of an endearing, but dejected-looking little subadult male mistletoe bird (who didn't quite have all his adult feathers and was sitting fluffed up in the cold). Despite his dejected appearance, and the cold surroundings, I wanted the painting to have a warm and endearing feel to it, similar to how I felt when I first saw and photographed the little bird. So instead of using cool blue-greys, I decided upon warm, but less saturated red-brown tints and beiges for the background.

My process to paint the background was as follows (please see attached work in progress time-lapse video rendering):

I started with my standard graphite sketch (including all the detail necessary to keep parts of the painting in proportion)
I then worked on the focal point (the bird and associated main branch), to ensure there were no irretrievable errors before moving forwards. I find that if I have problems making the focal point work the way I want, at this stage I might start the painting over again. It's a lot more difficult starting again if you ruin the subject after you've completed most of the rest of the painting!
I outlined the background details (over the graphite sketch) with a neutral pastel pencil (in a darkish colour that will show through lighter overlying layers).
After this I sparingly added the rough details and background colour for the far-away background and branches. In this painting I used pale to mid-tint neutral greys, beiges and creams for this task, and I didn't put it on very thickly.
I roughly blended this far-away background to completely cover the underlying paper (but not fill the grain so much that it wouldn't accept any further pastel).
On this base of somewhat variable background shades all blended together, I added further rough 'painterly' strokes of darker pastel to give the impression of branches with some texture in the background (I don't typically want my backgrounds to appear air-brushed - which would make them too stiff).
After the blending I mentioned earlier, I could still see the light outlines I'd done with pastel pencil underneath this overlapping layer of pale blended pastel, and I was able to use the outlines as a guide for adding the detailed shadows to the background branches.
I then worked up these branches with details from shadows to highlights to create form and texture. Branches that are further away should be blended more thoroughly (have 'fuzzy edges'), and closer or foreground branches should be given sharper edges.

How do you approach painting subjects with multiple layers and depth? Do you work all the layers at once, or do you work the background either first or last in the painting process?