Your problem is poster-child case for why you should use thin provisioning. Make the filesystems as large as you want, and let actually usage of the filesystems determine where the space is used. When you then realise that the 6TB XFS volume was too large, remove stuff from it and run fstrim on it to release the free space back to the thinp pool where it is now available to be used by the other filesystems that need space. No need to shrink at all, and you have an extremely flexible solution across all your volumes and filesystems.
And if you want to limit an XFS filesystem to a specific, lesser amount of space than the entire size it was made with (after releasing all the free space), you could simply apply a directory tree quota to /...
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