David, you have a sharp eye for big details my friend… I don’t usually go about painting parts while still attached to the sprues, but Zvezda has their 109 G designed to have the engine exposed as you can see in the photo, and the 2 cowling halves are easier (at least to me) to paint while still on the sprue tree and simply remove them only when all engine parts and bits are in their proper place, since they are the last detail to be assembled in that area of the model. The sprue connectors are thin so cutting, sanding and painting the small bare plastic visible after should be effortless. In the end the handling of those parts is a lot easier and safer. Any kit part that requires one or more parts attached so has to become one piece, like for example upper and lower wings, etc, those I go the usual route, painting only after all is completely assembled.
BTW, your diorama will take a lot of a number of things…Time+Research+Space to display it, mental health, etc, etc ;-). Looking forward to it, will follow your thread with interest