I personally go about it like this:

1. inbound/fba/2024-03-20/inboundPlans for inboundPlanId
2. inbound/fba/2024-03-20/inboundPlans/{inboundplan_id} for shipmentId
3. inbound/fba/2024-03-20/inboundPlans/{inboundPlan_id}/shipments/{shipment_id} for Shipping info and FBA id (shipmentConfirmationId)
4. inbound/fba/2024-03-20/inboundPlans/{inboundPlan_id}/shipments/{shipment_id}/boxes for info about how many boxes and their contents.

Honestly to me the new endpoint seems unnecessarily difficult. I don't see how the inbound plans make it better.