Hi there!
I found this thread, and wanted to let everyone know about our application, called Wholster, which allows for true partial payments to be made on draft orders using the payment terms features.
You can check out an overview of how this works here:
https://docs.wholster.com/article/payment-terms-overview/
For more general information about the app, please visit the app listing page in Shopify
https://apps.shopify.com/wholester.
If anyone is interested in this solution, please feel free to reach out to us any time at support@gist-apps.com.
Hope this helps, and looking forward to hearing from you!