For the time being, if you are writing a lot, I recommend making projects, and putting each section of the writing in a single note in a project. Eg. 1 project might be a chapter, and the notes the different sections.
We are working right now on collapsing. It will not be in our next Agenda 19 version, but is planned for Agenda 20 some months afterwards. Definitely coming though.
Even when you can collapse sections, it still is unlikely to be as good for long form writing as some other options. We didn’t really design it for that, and the multi-note timeline that Agenda has is perfect for shorter notes, but less good for your next novel! 