You can probably find some long-form tuned models on HF. I've had decent results with QwQ-32B (which I can run on my desktop) and Mistral Large (which I have to run on my server). Generating and refining an outline before writing the whole piece can help, and you can also split the piece up into multiple outputs (working a paragraph or two at a time, for instance). So far I've found it to be a tough process, with mixed results.