Thanks for reaching out.
You can set up a rate from within the Shipping Profiles to allow either the total weight or price of the order to display the corresponding shipping rates. I can definitely see how it would beneficial for Shopify to allow you to control both the weight and price for the free shipping rate to appear in checkout. With that being said, I can provide your feedback to our developers as a suggestion.
We do have third-party applications from our Shopify App Store that can assist you with creating this rate. I wanted to provide you with a couple of examples, feel free to check them out:
Let me know what you think.