Home DIY

How to explain this....

Background:

• we are having this room skimmed
  
• the french doors are recessed and there is a wood panel recess(?)/surround
  
• the wall edge of the surround is currently flush with wall, skimming will raise the surface height
  
• to mitigate this I'm fixing a 4mm trim (see attached pics)
  
• to finish we'll add another thin, but wider trim
  
  (birds eye view)
  
  (door frame)
  
  

Problem:
The top strip of the surround is currently too low in the middle in relation to the wall above. Idk what is out of level, the wall, the wood or a bit of both.
As a result when I add my 'raising strip', then there will not be a lip in that middle section to plaster up to.

Question:
What is the best way to fill that gap?

Ideas:

• put a cut off 4mm strip in the middle and then fill it as best I can