If I understand correctly you’re trying to make a GraphQL/query request? I am not proficient in JSON / Java Script but my company specializes in a software site called Make.com. If you haven’t heard of it, it is an automation and integration software that has modules which complete all the requests in the background and allows you to submit all the front end requests in a module format that you can customize according to your preferences. Anyway, through there it is possible to have an item created then have an update be sent to that item in the same automation. Which from your perspective I believe would still be two separate requests, but with Make.com those can be combined into one automation.
unfortunately you cannot in this case because you need the item ID from the create_item for the create_update. While you can create an item with column values, you cannot create one with an update. You will need to perform two mutations, one to create_item and return the item ID to then create the update on.