I don’t recommend going to master/subs unless:
- your document is intended to contain a lot of tables and images
-or your document is a kind of contract made of selected clauses officially approved by top management (to be extracted from a libray and used unchanged)
Today computers, even laptops, are powerful enough to manage “simple” documents up to ~800 pages. By “simple”, I mean containing mainly text.
There are ways to improve efficiency. The most important is “styling”, i.e. refraining from formatting your text directly with commands (for bold, italic, …), from spacing with empty paragraphs, spacing with spaces or tabs. Styles are your friends and you get a dramatic boost in responsiveness by adopting them.
Using master/subs requires much more discipline than a single document if you want to avoid discrepancy or formatting conflicts between the master and its subdocuments.
I would not use the track changes features to suggest changes. Suggestion should go to comments. Track changes is there to show what a reviewer corrected. When you discuss future paths for the document, do it in the comment without modifying text (if text is correct and makes sense in the current state of the discourse). Preserve present consistency. Remember you can copy from a comment to paste into the discourse.
Using track changes also requires some conventional strategy between author and reviewer(s). You can’t keep tracking across several iterations. You must agree on “checkpoints” where you accept or refuse the changes so that you reset the change history, thus creating a new step in the editing history of your document.
You may not be aware of File>Versions which allows to save a linear history of “versions”. You can the rollback to a previous step (but you lose later stages unless you save under a different name to not overwrite the current document).
They are saved in the edited document: when working on a sub-doc in this sub-doc; when working on the master stuff, in the master. But, I have not experimented to see what is displayed in the master when sub-docs have been modified. Anyway, I don’t think that accepting changes in the master will cause change acceptance in the subs.
IMHO, change acceptance must be handled separately in each document, master or subs.
Considering the methodological difficulties induced by master/subs in track changes context, I suggest you think twice before going for it. You’ll gain a tremendous performance factor by following a very strict and methodical styling policy.