How to simplify navigation bar content in gitbook?

Question

git

There is a mountain of documentation that is being rolled into a gitbook.
This mountain has a heavily nested structure, incl. subheadings up to level 6 and there are sooo many of them. All this is included as is in the root SUMMARY.md. As a result, the navigation bar cannot be used. What ways I see, with what I actually came to ask:
1) We need a normal plugin for folding subtitles, as it is in Doxygen, for example.
2) Or you need to have only the first level headings in the root SUMMARY.md, and only they are displayed, and all subheadings are in SUMMARY.md in subdirectories. But to do this is not very successful, if the page is not specified in the root summary, it is not generated. I suspect that it is impossible to achieve this by standard means, which means that some kind of plugin is needed again?