Best way to inject variable content into post body

This is more of a question of execution, so I’m mostly asking for ideas on how to achieve this.

I’m building a new site for an existing blog.

On blog post pages, I want to be able to automatically inject content into the middle of the post, effectively breaking up the post a little.

This content would be pulled from a repository – perhaps custom post type? So I could create various types of content (text ad, graphical ad, a sign up form etc).

So for example, lets say a blog post is 10 paragraphs long.

I wish to inject a little “ad” in the middle of the post, which promotes another page or post in the site. Or perhaps it will be a signup form, for readers to subscribe to the newsletter. So kinda like injecting an advertising banner, except the ad is an internal one for the site itself.

This would of course apply to ALL blog posts on the site, although it would be even better if you could select particular category posts it would apply to (so it would “inject” into posts under Category B, but not into posts under Category C, for example).

Then it would probably need to have some sort of “limit” – it wouldn’t inject into posts if they have fewer than X number of words (because it would look silly).

I’m assuming the best method would be to:

1. Create custom blog post template page under the Child theme
2. Code the file with a custom script/shortcode that “publishes” the content from custom fields or custom content into the blog post after X amount of content?

Am I overthinking this? Is there a plugin that already does this?

Any ideas/comments welcome.