If you are using the REST API then you are pretty much constrained to a single new record creation per API call. If you are getting 2 API requests/second throughput as expected then you get an idea as to the time it will take to create a draft order set. In code I usually define the method that handles the API request/response pair, pull in an array or list of records I need to create, iterate each record, passing it to my API method. Not a ton of extra coding really. Let's say you have 1,000 draft orders to create. Given the API call rate, the whole routine should finish up in under 10 minutes.
You could also use a low code tool like Integromat or Zapier to automate this step for you.
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A kinda open-ended question, but if you are just initially looking at this task if it were me I would:
In the event you are going the self-developed route, it's all up to you. As to what development environment, language, tools, etc. you are going to use. If it's a one-time integration then it'd make sense to setup a test development shop to use. Configure the barebones requirements, drop in some data, and have a private/custom app registered against it. That private/custom app is where your integration will live.
Here's Zapier --> https://zapier.com/apps/shopify/integrations. I've looked at it in the past and it's a good service depending on what you are looking to do and how much you are willing to spend. I don't work for Shopify, so I'm not sure about any form submission!