Dr Dog wrote:

The length of these notes can run into 3 or 4 thousand in a few hours of
writing, so I really need typewriter mode (so no Bear) and folding - I
often find myself writing in sections (so no Ulysses or Scrivener).

The whole philosophy of longform writing in Ulysses is writing in sections, aka sheets.

Here are a couple of intro/comparison articles you might find useful:

https://zapier.com/blog/ulysses-markdown-writing-guide/

https://zapier.com/blog/ulysses-scrivener/