Wow, this is almost exactly what I was planning to do for my site. For another small project, I wrote a tiny shell script as a makeshift "bundler" (just embeds the CSS and JS inside the HTML) with the goal of also being able to serve the unbuilt files locally:

    sed \
    -e '/[/][/]script/r index.js' -e '/[/][/]script/d' \
    -e '/    <script defer src="[.][/]index[.]js"><[/]script>/d' \
    -e '/[/][*]style[*][/]/r styles.css' -e '/[/][*]style[*][/]/d' \
    -e '/    <link rel="stylesheet" href="styles[.]css">/d' \
    index.html

The HTML contains something like this:

    <link rel="stylesheet" href="styles.css">
    <style>
    /*style*/
    </style>

and the script just deletes the <link> tag and replaces the /style/ comment with the contents of styles.css. Definitely not my finest work but it worked well enough.