Hi Anna
If they're not prepared to purge the data and Basis have no other configuration changes that they can apply, then I think the next course of action is an investment in memory.
Have a chat with the Basis Team and get their recommendations on what would be required.
In addition, you could log an Incident with SAP for them to investigate, they may have some notes / recommendations on how to resolve without investing further.
Regards
Arden