Unify the legacy ERP and the operational data into one model
Orders, customers, and historical financial data live in the legacy ERP. Shipment status, ship windows, validation outcomes, and customer-specific rules live in the modern operational system. Neither side, on its own, can answer "did we hit PIFOT this week."
Pull both sources into a single query-able model that's authoritative for SLA reporting.
An ETL service reads from the legacy ERP through its own data layer and from the operational SQL Server, lands the data in a clean star schema with eleven dimension tables and seven fact tables, and exposes pre-aggregated views for the queries that run most often.
One source of operational truth that reconciles back to both sides and is friendly to BI tools.