They enabled Multi-AZ to fix read load

An RDS PostgreSQL instance is CPU-saturated. Roughly 85% of the query volume is reads from a reporting dashboard; the remaining 15% is transactional writes from the application.

The team enabled Multi-AZ, reasoning that a second instance would share the load. Cost roughly doubled. CPU on the primary is unchanged. Dashboard queries are as slow as before.

They now propose enabling Multi-AZ in a second Region as well.

Explain precisely why Multi-AZ changed nothing about the symptom, what they should have done instead, and what the correct fix is given that 85% of load is reporting queries.