@T_Pover Thanks for looking! I really do appreciate the kindness of people on here.
I'm a total newbie with accessing the API (currently struggling with navigating the API file in the SDK)
If I can find a way to point towards this specific list then I imagine it might be possible to use Python to create a list of all the line subcategories then use a for loop to delete each one? I'm thinking I'd approach this using RevitPythonShell or PyRevit.
Does that sound sensible?