I found a few old posts that there's no direct "trigger on variable change" but is there any good work around?
Only by adding an Execute a Macro action every time you change the variable.
Here's an untested idea: When you change the KM Variable (or maybe instead of using a Variable, use a file), you could create a file in a specific folder that is KM watched folder. When the file is added, the macro will be triggered. It can then delete the file.
See Folder trigger.