Ask HN: End-to-end encrypted online spreadsheet tool

I'm trying to identify end-to-end encrypted alternatives to the tools used by an NGO I work with that deals with sensitive data.

Tools for cloud storage ([Tresorit](https://tresorit.com/)), email ([Protonmail](https://protonmail.com/)), instant-messaging ([Signal](https://signal.org/)), collaborative text documents ([CryptPad](https://cryptpad.fr/)), collaboration between teams ([Keybase](https://keybase.io/)), hard drive encryption ([VeraCrypt](https://veracrypt.fr/)) abound, they're free, easy to use, secure, and overall amazing.

The one obstacle I haven't been able to tackle in several days of searching is an alternative to Google Sheets, a secure online spreadsheet with end-to-end encryption and collaborative editing, like what CryptPad (https://cryptpad.fr/) does for Google Docs.

Is there a ready-made alternative, or do I need to build a custom solution?