Probably Astro can do what you want, or Grav CMS or Tina or Ghost etc. There are also a bunch of hosted commercial "headless" CMSes (disclaimer: I work for one) that let you more cleanly separate content and presentation with easy to use editor experiences and good devex for static builds. They often have free plans, but IMO you don't really need something like that, just a simpler Markdown based system with some minor custom coding for embeds and such that you can self-host.