Hello Tony,
My colleague is correct. Because both writing (for importing a CSV file) and reading (for dumping all entries to a file) need to go through the E2E encryption, they need to happen on the frontend.
Doing this in the browser is technically possible, but it would take multiple days of engineering to get it done.
We have other clients that also want this feature. Would your organization be interested in sponsoring part of the development effort so that we can implement CSV import/export of E2EE data in REI3? If we find 2-3 interested parties, we could prioritize this feature and get it done soon & cheaply for all involved.