I have seen someone asking the question but looks like there are no solutions. Would like to describe why this is a needed functionality. Using an ERP system as the source of truth for high volume enterprise clients is a MUST functionality to do a fast synchronization with different sale channels, Shopify in the actual case.
Using the SET from the rest API is OK to synchronize the inventory quantities, but you need to do one item per single call, so the bulk method is a lot faster. If I'm not mistaken you can update 100 items per second. (please correct me if I'm wrong). The problem with adjust is that we need to do a previous call to fetch the inventory values we want to update. After we need to find the delta = (Our quantity - shopify quantity), then use this as value for avaiableDelta. Really a bad way to do it as during the time we fetch the data, manipulate the values and do again the call to shopify with adjustment, it might happen that some of the inventory is already updated (customer bought something, etc...), which mean desynchronization, but as well not as fast as doing a single call.
I really hope to see this feature added so high volume enterprise clients will be much happier. Thank you and looking forward to it.