Here is information about the Developer Tab. It depends on how much time and effort you want to put into your template up front.

A userForm is probably the best option. It is certainly a good one.
Create a Simple Userform

I would suggest putting the individual parts into AutoText or Building Blocks and having your UserForm insert them into the document as needed. They should be created using Heading Styles that are used to create a Table of Contents. Then your userForm can insert a table of contents field (possibly also in AutoText if you customize your TOC) and update it as it finishes.

The AutoText/building blocks can be stored in your template but don't display in the document until/unless called into the document. They are not any part of the document's size. For me, AutoText is much easier to format and edit than trying to directly insert contents using the userForm (vba).

Automated Boilerplate Using Microsoft Word
How to create a table of contents in Microsoft Word

If you simply want to use the Building Block Gallery approach without the UserForm, I would suggest exploring Greg Maxey's tools (free).
Building Blocks & AutoText

For an example of a simple custom gallery, take a look at my 2003 WordArt Add-In for 2010-2013. While that holds WordArt, you can certainly put complex text in your template's galleries and those galleries can easily be attached to QAT buttons in the template. Note, though, that the gallery buttons all look the same and are cryptically named.

You could also add a custom ribbon to your template to allow much more flexibility (or restrict user choices). Customize the Ribbon (It doesn't take rocket science). That could include one or more userForms and custom galleries.