So... it's at this point of the project that I come to realize I've probably made some pretty big authoring mistakes. Not so much my inexperience in Substance... but moreso a few fundamental art errors.
These, I think, are the basic steps I was missing at the start of this project:
1. Concept art (Fully defining the scene and all ideas within it.)
2. Blockout (creating the basic shape of the environment and establishing scale.)
3. Silhouette (altering the blockout to be more aesthetically pleasing and coherent.)
4. Research & Reference (collating relevant images/ information about the scene.)
5. Moodboarding (establishing overall theme and colour palette.)
I'm going to take a step back from making assets and focus on greyboxing the whole scene now.