@s.522
some folk work on pre(chapter)split books
some folk would rather work on a huge, long file
I prefer the first. Less damage when things go waaaay off in left field
It does not matter WHEN you create the TOC if using H#s as TOCgenerator markers, as long as they are all present Splitting will adjust references.

I use the Clip Editor to create common tags (some in
wrap mode) and strings used in code view. The frequent used ones are on the
Clip bar One is the
Sigil Split Marker (ctrl-enter) (see circled items in image) and use
Split at Markers (F6) when ready


Clips can be placed into a Search or replace box as well as the body text. Sometimes I build a replace compounded from clips
Image shown is a
non-traditional toolbars layout (My goal: minimal mouse travel to common used icons)