I don't use a static file "CMS", but I use my own generator[1] which I made because I wanted something really fast. If you want a flat file cms, then a Google search will throw a lot of them. Kirby is the most popular, IMO. But it's not FLOSS. PicoCMS ticks all those boxes though.
I wanted a "full" CMS and a GUI, and it looks like finally I got one now. I hadn't seen another static generator that had a GUI of any kind (that seemed to be supported.)
[1] https://github.com/pawandubey/griffin