2 years ago I faced a similar issue. The only issue I had moving data from the boot drive to an external drive was in dealing with files managed by apps rather than just the Finder.
I determined that moving the contents of my Apple Music and TV libraries to an external hard drive would create enough free space on my new system that I would be comfortable with. I looked at the ‘Media folder location’ entry under the preferences for each app and then copied the portion of the hierarchy in the Home folder to the external drive. I then started the Music app and replaced the Home folder reference to the external drive. Unfortunately, at that point, I saw the media files being again copied from the Home folder to the media folder on the external drive, resulting in duplicate files.
I then searched for some outside help, via the book ‘Take Control of Your Media Apps’. A section of the book on using external drives gave a cookbook for letting the app populate the Media folder. Here are the steps from the book for populating the Music Media folder (populating the the TV folder works the same way:
“Go to Music > Preferences > Files.
If “Keep Music Media folder organized” is not checked, check it. (After you’ve moved your files, you can go back and uncheck this option if you so desire.)
In the “Music Media folder location” section of the Files preferences, click Change.
In the dialog that appears, navigate to your external disk or network volume. Continue navigating to where you want to keep your Music Media folder (this may be at the root level of the disk). Click New Folder, name this folder Music Media, and then click Open.
Click OK to save your changes.
Choose File > Library > Organize Library.
In the Organize Library dialog, select “Consolidate files” and then click OK.”
Take Control of macOS Media Apps (1.5)
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So I first erased the contents of the appropriate folders and followed the steps above. After doing a random check on some files (Pick a tune in the Music app, right-click on it, select ‘Show in Finder’, and confirm that the volume is the external drive),. After the verification was done, I backed up both the external drive and boot drive, let everything sit for a few days, and then got rid of the original media folder on the boot drive.
Followup note: I recently reduced my data on the boot drive by moving some more large collections, including the iCloud Photo Library to the external drive. This went smoothly, although the Photo Library took several days to successfully sync with the cloud version. Of course, I also needed to modify apps involved with referencing or managing the collects to point at the new locations of the active folders.