Hi there, this is a tough one to write out an answer to because it's a lot of info : )
Generally, our creative custom form is fairly basic and we take a deliverable = project approach (programs = campaigns, events, anything that requires many assets) and projects are a deliverable or set of like deliverables (a blog post or a set of banner ads, etc.).
We have 3 creative team PMs that get requests from other business units and convert them to projects, and for marketing/internal creative needs the project owner creates the project but uses the same project template and assigns to studio PMs to process/assign to a resource.
Happy to hop on a call in the near future to explain more in depth if it'll help give you ideas.
If this helped you, please mark correct to help others : )