Alternative to `render_individual_file`?

I’m currently in the process of upgrading my site from Middleman v3.x to Middleman 4.2.1. I have a custom helper method called manual_render that is used to render and shrink header tags by one (ie: <h1> becomes <h2> etc.).

In version 3.x I was able to use the render_individual_file method, but that seems to have been removed in v4.x.

Is there an alternative method I can use? Something that returns the equivalent content of render_individual_file? I’ve tried using render and render_file, but those return the rendered html content of the file. I want to just get the markdown content of the file.

How should I go about this?

Thanks in advance!