This is not a trivial ask of Microsoft. Create a new subcontract PLANNING workbench where a user can see ALL the implied subcontract PO's that would be created from planned production/batch orders. Zero change in setup, you still add a Service item to the BOM/formula of type Vendor and optionally tie it to a Routing step. The workbench would permit GROUPING and SPLITTING of the implied Subcontract PO's (Wow!). Selecting all lines for a Production/Batch order or set of Production/Batch orders would ONLY firm Production/Batch orders IF all subcon lines had been checked as OK (AFTER optional Grouping/Splitting). Errors would highlight any missing rows. The resulting PO's would have to be referenced back to their Productions (and vice-versa) on a many to many relationship rather like Mark lines, except these are NOT inventoried items (so reserving makes no sense).

Now we can:

1) Actually plan ahead for subcontract

2) Group a bunch of subcontract requirements (same Item/Variant or multiple) on a single PO - Oh my!

3) Split subcontract across multiple subcontractors for same Production (leverage the same setup as stock items?)