Would be great for us to use in our project.
If you are interested, you can evaluate the toolkit and make any modifactions you please. The free eval last for 60 days and includes demos as well as the source code.
Disclaimer: I am an employee of this product
We just released a developer library/api for annotation product Markup Hero. It's quite slick and easy to implement. Here is a demo of it in action with Notion (this is just a demo, Notion hasn't integrated it... yet). But we have a handful of SaaS companies integrating. Quite flexible and clean.
30 Second Video Demo: https://youtu.be/miB3yD1xlTI
Quick Interactive Demo: https://markuphero.com/try/notion-annotation-api-demo.html
Info on the library is here: https://markuphero.com/integrations/annotation-api.html
Documentation for the library is here: https://docs.markuphero.com/#summary