We use https://react-page.github.io/ (also MIT licensed) extensively at my startup; it attacks the same problem, and it's been incredibly effective (and hackable!).
Generally speaking, owning your own CMS data, in your own database, with a well-documented JSON data format, and adding the ability to take any React component you've written (that itself may interact with your own data) and make it not only reusable as part of a content editing system but also WYSIWIG, opens up a huge number of opportunities - including adding your own logic to transform content before display.
https://builder.io is another alternative that's very effective at the adapting-custom-components-to-WYSIWIG side of things, but does keep the data in its own cloud storage.
I'm really excited to see innovation in this space, and I'll be following Puck closely!