Don’t know if you read all of the posts in this thread but I posted on 3/21 this
"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 
- Anyway, we are not affiliated with them, I just tested out several apps and this one stood out. It starts at $29/mo but pays for itself. - good luck "