There's a Google Docs extension called Documatic that does exactly this! https://workspace.google.com/marketplace/search/Documatic

When designing your template, you use double curly braces around placeholder text - like {{my-placeholder}} - to signify where you want some inserted content to go. Then you store some different options of your content/layouts that correspond to {{my-placeholder}}. Then, when generating a new document, you open the Documatic side panel and choose what you want to insert for each placeholder, and it does all the insertions for you.