C. Use Big Objects for cases older than 2 years, and use a nightly batch to move them.

Big Objects in Salesforce provide a way to store large amounts of data that is accessible within Salesforce but doesn't count against the regular storage limits. This solution is suitable for archiving millions of cases while keeping them accessible for on-demand querying.
A nightly batch process can automate the movement of cases to Big Objects, ensuring that only the last 2 years of cases remain in standard objects for operational reporting.
D. Use Heroku and external objects to display cases older than 2 years and use the Bulk API to hard delete from Salesforce.

Heroku can be used to store large volumes of historical data externally. By using external objects and Salesforce Connect, service agents can access these cases directly from Salesforce's interface, ensuring that the data is available on demand.
The Bulk API can be employed to efficiently move large volumes of data out of Salesforce to Heroku and to manage the deletion of records in Salesforce that are older than 2 years....