Very interesting use of ChatGPT and prompt engineering. For your problem, summarizing a large document, splitting the document into smaller parts is indeed the way to go. I also had problems myself with operating on large documents. In my case, I had an insurance policy that I wanted to extract information from.

My solution: use the OpenAI API to convert the document to OpenAI's embeddings and saving those embeddings to a vector database. Then, use similarity search on the database to find chunks of the document that might be related to my query and pass only those chunks to GPT for the information extraction prompt.

I plan to create a guide on how to tackle these problems after I consolidate my findings.