First, stop messing with the header. Use the StyleRef Field instead. That is what it is designed to do.

Second, consider using AutoMatic Numbering tied to your Heading 1 or another style.
How to create numbered headings or outline numbering in Ribbon Versions of Word by Shauna Kelly
If you want to add a chapter heading, apply the style. If you add in the middle, it automatically updates everything after.

This also gives you a good start on your Table of Contents.
Generating a Table of Contents
Why use Microsoft Word’s built-in heading styles? by Shauna Kelly

If you really, really, want to do this the hard way, you could save your number as a custom document property.
How to use a single VBA procedure to read or write both custom and
built-in Document Properties

To me, is seems like you are trying to re-invent the wheel. This stuff is built into Word.