Why not leverage ES template strings? From what I can see it would support the same features (interpolating global JS, passing individual helper functions in, etc) natively.

I've built a teeny tiny template engine around this, allowing me to consume any string at all, and treat it like an ES template string: github.com/tomhodgins/jsts-engine

function jstsEngine(string = '', environment = {}) {

  return new Function(
    ...Object.keys(environment),
    'output={}',
    'return [`' + string + '`, output]'
  )(...Object.values(environment))

}

So I wonder what your template engine offers that ^ that template engine wouldn't support.

Here's an excerpt from a text file where I use it:

<ul class=toc>
  ${

    galleries
      .map(gallery => `
        <li>
          <a
            href=${site.url}gallery/${helpers.slug(gallery.title)}.html
            style="background-image: url(${helpers.randomArrayItem(gallery.images)});"
          >
            <span>${gallery.title}</span>
          </a>
      `)
      .join('\n')

  }
</ul>

So it can be used to template any language as well, here I'm templating HTML. Where you see things like ${gallery.title} that's reading a value directly, where you see things like ${helpers.slug()} or ${helpers.randomArrayItem()} you're seeing me run content through a JS helper function I also made available to the template engine. And as you can see here, it's all nestable too. You can interpolate inside the things you're interpolating too.