[preya2k]

What OP is building is not a typical "flat file CMS".

Flat File CMS are typical CMS systems (often times written in PHP) that run on the server, but use files (often Markdown/Frontmatter) as their data backend (instead of a DB like Wordpress, Drupal, etc.) – if you're looking for a really nice Flat File CMS take a look at Kirby (https://getkirby.com) or Statamic.

What OP is building (I think) and what others like Netlify CMS and Tina CMS do, are Frontend Applications (typically SPA) that output a set of content files, which can then be fed into a static site generator (like Next.js, Astro, Hugo, Jekyll), which will built a website from it. Often these content files live on Git – so the common interface between a SSG and the "static CMS" is often Git (or a local folder on your file system). So it's a "smaller" concept than flat file CMS. Typically these "static CMS" only care about content and have nothing to do with templating, etc.