For anyone else who comes across this - I resolved this be enabling the Free Shipping option in Configuration > Sales > Shipping Methods but the trick was adding a Minimum Order Amount.
I put this at a ridiculously hight amount, at 5000 and then the Cart Price Rule I created for > $100 was working.
I'm on 2.3 and ran into similar issues with free shipping:
- Free shipping and Flat rate don't work together
- Free shipping shows on the checkout page even when it shouldn't (one page / klarna checkout)
- When using Cart price rule as suggested by Magento docs, then two free shipping options are shown on checkout page
Finally I got so fed up I put in a work around - I imported a tablerate.csv with the rule set for free shipping:
Country,Region/State,Zip/Postal Code,Order Subtotal (and above),Shipping Price
Now shipping is free above a certain amount and there is only one shipping metod showing