Solutions
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tryhardcodemaster's post in ZFS Mover between two pools. was marked as the answerWriting to /mnt/user/media.
I figured out the issue. The minimum free space being automatically calculated was 1.5TB, but the cache pool is only 500GB. If it set the minimum free space down to 100GB, it would start writing to the cache pool for new files, rather than directly to the data pool.