I will get back to you later on this. See
this thread. where I lay out two methods, one using AutoText, and the other using bookmarked parts and vba. Look into AutoText and other building blocks in the meantime.
Automated Boilerplate Using Microsoft Word
More than 15 years ago I uploaded a sytem for doing this, but it would require some updating and I've lost the password for the vba. It is the letterhead system on
my downloads page.
See also
A Global StyleSheet in Microsoft Word?