Possible ideas for clients not on metrc would be to simulate some of metrc process. 

For the PO, create PO- instead of current way, you would have an interface to create the main part similar to the small option on the bottom of the metrc add new inventory page. Once you filled that it, you would go to the mapping page, like for the metrc api. it would have all of the parent items for that vendor and fields where you could enter quantities, similar in look to the update po fields/boxes. This would create the new subs, and you could also add the sub attributes on this page, so you don't have to go back in to each sub on a PO and add sub attributes ie strain, testing, etc.

If the parent doesn't exist, there is a link where you can add new, just like currently on the api mapping page.