What term is the EASA equivalent of the DER in the FAA safety assurance process?
The closest EASA equivalent would be the Design Organisation Approval (DOA). According to EASA:

A Design Organisation Approval is the recognition that a Design Organisation complies with the requirements of Part 21 Subpart J.
Privileges: A DOA holder can

Perform design activities within the scope of approval

Have compliance documents accepted by the Agency without further verification

Perform activities independently from the Agency