My name is Zack! I saw your post and was wondering if you've ever heard of "Stitch Labs"? It's a 3rd-party app that allow you to manage the inventory of multiple stores, all from one location. They're definitely worth looking into!
Another app, called "Brightpearl", can do this as well; take a look at them both, as I think these are up your alley :)
All the Best,