Have you seen this: https://github.com/front/storypage
I know there are some trade offs on the waty gutenberg is built (they isollate it inside a common react/redux) app, but they provide an "external" API (ala backbone style). For exemple, all async calls are wrapped around a global function on the WP namespace (If i'm not wrong) window.WP.fetch (or something like). On the github.com/front repos there are more examples, and also a mantained gutenberg.js package.
The only reason I still have an eye on it, is that gutenberg has lot's of resources inside (400 contributors), 9000 commits... this is something that only can be done by a paid team (wordpress.com) with lot's of resources.. There is also a rising ecosystem of "plugins" (ex: https://github.com/thatplugincompany/coblocks)... To have something like this around something like plone it's really a dream!