See Automated Boilerplate Using Microsoft Word.

It can be done but you are right, it isn't simple. There are probably multiple ways to do what you want.

I would start by making coded AutoText entries for your variations.

I would then have a UserForm (if you can use macros) or a form drop-down list if you cannot. Create a Simple Userform

Your userform or drop-down gathers essential variable information.

Then you have locations in your document template to be populated by the AutoText entries by your UserForm. If you instead use a drop-down list you would use IF fields to test which conditions were met in your list and then use AutoText fields to pull in the AutoText entries.

Simple? Nope. But, it isn't rocket science either.

Another page to look at: Repeating Data (Populating Multiple Like Fields)
That page will give you a look at the multiple ways Word has to store variable information.