How can I off-load iCloud Drive from Macintosh HD to an external disk?

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I've got a 2013 Mac Pro with a 1TB internal SSD that I imagined would be ample for the next three years at least -- but that was before my wonderful spouse gave me a Sony a7rII mirrorless camera and I started shooting raw. The photo files are sooo big! I'm putting them on iCloud so I can edit on my iPad Pro and everything is working beautifully, except both iCloud Photo Library (334 GB) and iCloud Drive (275 G, of which 45GB reside on my internal SSD) are rapidly swelling. I've successfully moved the former to attached storage and would LOVE to do the same with iCloud Drive's 45GB. However, I can't find any documentation of how to move it -- not without moving my entire home folder. Any suggestions?