Hello Carl,
We built our own RTE at the Guardian to use as part of our editorial tools, partly because as you pointed out existing solutions tend not to be particularly extensible or lightweight.
It's open source and available at:
The minimised core (without plugins) is 38KB, with no dependency on jQuery or other libraries. It's used in production in the new Guardian CMS.
I actually tried to turn it into a Web Component, but that was literally the work of an hour or two during a Web Components workshop in London, so the code is fairly rubbish and hacky. Basically it proves that it's possible, but I would try and rewrite it cleanly from scratch if I wanted to go further:
There are some interesting challenges around how to coordinate multiple elements that form a whole (e.g. the editor + the toolbar buttons). We struggled with that in our AngularJS implementation as well, but I still believe it's all doable.
Would love to hear if you're interested and have a go at making it a Web Component, I'm sure a lot of people could find a use for it!
Feel free to ask any questions by email or even better, via the Github issue tracker.
Best,