The collaborative encryption is exactly what is solved in CryptPad.fr and at the core of the storage model. This is actually key because it's not the only problem you will get once you try to add encryption. You'll realize everything needs to be rebuild with no database on the server side, otherwise you will get only partial encryption. Encryption needs to be built in first and then the rest should be added.
Concerning user experience, do not confuse design and user experience. Yes some design is needed to improve cryptPad, but the user experience starts with making it super easy to start to share something. In the case of bloom you first needed to register which is not needed on cryptpad to start working collaboratively.
Note that cryptPad storage is a bit like S3 but with e2e encryption and collaborative documents built in.. so you could build your UI on top of it, but e2e encryption means everything needs to be done from the client side, not from the server side.
There is also a roadmap. I will contact you. We can meet. We are next to Beaubourg.
As being a huge topic, federation is planned for Q3/Q4 2020.
I checked cryptpad, but I feel it's really lacking focus on user experience.
About e2e encryption, it's planned for Q4 2019, but collaborative encryption is really not an easy stuff. "Cryptography is a serious matter".