Purchase Order Histroy Limitation

We have the business requirement to process selective SAP purchase orders which grow in size to over 500,000 line items due to the PO history. However, the size of the PO has become problamatic regarding the amount of time it takes to run on our server. Our basis team has informed us the threshold for the PO history is approximately 30,000 line items. The scenario for this large POs is most prevalent for our CATS process where a batch job creates Service Entry heets from time keeping entries. A specific time sheet per contingent worker is processed at least 10 lines per week as orkers are asked to split time between lunch. The contingent worker charges time against 4-5 work orders in a given eek, which in addition to splitting time can be 25 line items per week per user. So this translates to 25 line items on the SES and 25 line items on an ERS Invoice. The current SAP limitation is forcing us to shorten the PO duration to 2 months instead of the anticipated 1 year life cycle. Also the processing time for approving the SES and processing the invoice takes approximately 24 hours because of the PO history. We need a resolution to at least make the PO last for a year, and would very much appreciate any insights on if anyone runs a similar process without encourtering our stated issues.