Good points!

> I don't see if the private half of the key is shared with the web app to decrypt the cypher text, or if the cypher text is sent to the key service which responds with plain text.

I checked out the ref for that encryption API. Seems like the service stores the key, and the API exposes encrypt/decrypt calls:

https://developers.google.com/workspace/cse/reference

So yeah, go trust that third-party provider, or if you have the technical skill, set up your own. Since most people would do the former, this is basically back to square one, and to call this "end-to-end" is misleading.

The idea of storing your keys on an Internet-facing server baffles me too. It will 100% get hacked sooner or later.