I am not a big fan of web apps, but this looks like it fits the org-mode use pattern of to-do lists and so on.
But there's no integration with formatting, stylesheets, or anything, right?
In 2017 or so I wrote the maintenance manual for an earlier version of this educational 3D printer:
https://www.ysoft.com/be3d
It was about 350 pages with lots of illustrations and diagrams.
The entire manual was a single giant Word outline. That meant all the headings, subheadings, sub-sub-headings etc. were automatically formatted for me, plus automatic generation of table of contents and index, all automatically linked to the structure of the steps in each chapter.
The first edition took me about 6-8 weeks, the second edition under a month.
Later on I spent 4 years working with DocBook XML and a professional documentation production system -- but I can state from personal experience that for a solo project, me and 1 engineer, Word in Outline Mode was vastly more productive and efficient.
To the client's amazement, the final stage of print-ready layout took part of one morning at the end. I took one of their leaflets, laid out in Word, removed all the text leaving only styles, and then merged that stylesheet with the manual.
Bingo, fully-formatted document, in corporate colours and fonts, with headers and footers and so on. Just a little layout tweaking, adding some page breaks and things.
The ability to collapse all the levels down to just headings is an amazing navigation tool: it's like zooming out to an overview, then zooming back in somewhere else, while the text remains readable throughout.