[unicornporn]

https://getkirby.com/ is nice, but not open-source. https://getgrav.org/ seems similar. https://bolt.cm/ is perhaps not as sparse, but the best flat file CMS I've found (SQLite).

[fbnlsr]

Kirby is really good. I've tried it a few weeks ago for a simple website (for which I would have used Wordpress) and it did the job perfectly. It's fast, efficient, has a great documentation and is really easy to set up. I have nothing bad to say about this.

[aargh_aargh]

I can recommend Bolt CMS for its declarative definition of content types and overall simplicity. (It's not limited to SQLite, though.)