what do you think about create Cart Price Rules to add free shipping method??
Magento Doc: https://docs.magento.com/m2/ce/user_guide/marketing/price-rules-cart-free-shipping.html
Step 1: Enable Free Shipping
Enable Free Shipping in your store’s configuration.
Complete the free shipping settings for any carrier service that you want to use for free shipping.
Step 2: Create a Cart Price Rule
On the Admin sidebar, tap Marketing. Then under Promotions, choose Cart Price Rules.
Follow the steps below to set up the type of free shipping promotion that you want to offer.
Step 3: Complete the Labels
Complete Step 4 of the cart price rule instructions to enter any labels that appear during checkout.
Step 4: Save and Test the Rule
When complete, tap Save Rule.
Test the rule to make sure that it works correctly.
Price rules are automatically processed with other system rules each night.When you create a new price rule, allow enough time for it to get into the system. Then, test the rule to make sure that it works correctly.