[Quoth]

It's like the old joke of tourist in Dublin looking for a hotel. The tourist gets a very long explanation and then the Garda (Irish Policeman) ends with, "But I wouldn't start from here."

Yes it a lot of work, but if you have chapters, then set next style for the heading style to be "default body text", copy & past heading, set to Chapter style, hit enter, and then copy paste all the source body text for that chapter to that line (in Unformatted paste mode so current style is used).

A 120K word novel takes less than an hour.

Also learn how to save your styles as a template in LO Writer or Word. I have three templates for three format/styles of epub and also each of those a more complex set of styles for PDF. If the template for the epub and the template for the PDF version use the same names, then copy & paste of entire document works, but the properties of the destination styles are used. Then all I have to do is change the page style applied to each section (as PDFs use many and ebooks only one)

I'm using Calibre 6.9 on Linux Mint 21.2 with Mate Desktop. As an experiment I copied a about 82K word docx to XP with Calibre 1.48 and converted to epub the system TOC and content looked fine in the Viewer.

I used Word till about 2014 then LO Writer on Windows, then Linux full time since Jan 2017 (Though I was using Linux part time in 1998).

I discovered to my own cost in time the problem of typing text into a Desktop Publishing program (which InDesign is). Even before Word or Open Office progressed to being good, I discovered that even if using DTP it was better to edit in ANY wordprocessor and re-import to DTP. I've not used DTP for years because it makes conversion to epub harder and PDF is best for paper publishing and can be produced without InDesign or other DTP easily unless you are working for a magazine or newspaper.