Sorry I know you said headless, but have you considered wordpress? Its unsexy, but the ecosystem is really well suited for this and hosting is dirt cheap.
Because its so popular and been around for so long, theres tons of free themes, plugins and videos which will reduce your support burden - plus your admin could get help easily as its not something you rolled.
Other than that, decap on gitlab is easy to run for free, and will provide the admin with a ui for editing content. Astro is also great and stable for this type of thing.