We ended up biting the bullet and used a paid app. Discount Ninja. I liked this app so much I made video about it here: https://www.afterdarkgrafx.com/combine-discount-code-free-shipping-shopify/
It is silly that Shopify does not offer this simple ability on their basic platform
I have the same problem too, and would like to add my vote. I have automation emailed discounts of 10% for abandoned checkouts, but i will then get emails from some customers saying that they then no longer qualify for free shipping, rendering the 10% useless. So then i have to spend time making a special code just for them, figuring out what the discount would've been based on their cart and getting to the closest %age.