Upload the doc to Gdrive in a folder named templates or something like that. Make sure gdocs is configured to covert uploaded docs to Gdocs format. Change font and line-spacing etc and then make multiple copies of your template. Every time you create a doc you open one of these template docs, edit it, rename it and move it into another folder.
https://support.google.com/docs/thread/260073437/custom-page-sizes-in-gdocs?hl=en

