"Off-loading" does not mean a paid upgrade but exactly what you already tried - moving files from the internal to an external drive and then linking it if necessary.
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@Fishrrman asked, what is the application in question?
How did you link the directories? With a simple alias or an actual symlink?
As an example, if you have a folder named "VendorDirectory" on the root of your external drive, you can create a symlink to said folder in your user's "~/Library/Application Support/" folder with:
ln -s /Volumes/YourExternalDrive/VendorDirectory ~/Library/Application\ Support/
What command is used to create a symbolic link/soft link?
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