What is a good replacement for the 'df' command that is broken on macOS Mojave?
The new command to purge snapshots is seriously faster and more well engineered than the previous local store. 

It does require a new syntax, so you’re correct, Apple has forced us to learn more and change. 

Joshua was in your situation as well and has a very detailed explanation:


How to thin your local Time Machine Snapshots on macOS High Sierra


Apple tools take a while to classify what’s purgeable, but that is getting better. 


macOS not showing the actual free space
Why do system information and du show drastically different numbers


If you don’t see immediate clearing of space, it’s a sign your disk has accounting issues and you need to boot to internet recovery or plain recovery on an external drive and repair the wrapper and container. 


APFS/High Sierra - `tmutil deletelocalsnapshots` not deleting snapshots