The easy way to do this is to move your entire home folder to the external drive. Apple designs their OS to have the user home folder portable, but everything inside that home folder really derives from the location / storage of the folder.
You can sometimes override the location of things like iTunes Music Library or iPhoto Library (the parts that don't cache down from the internet).
Moving bird and the sync data is one item that seems exceptionally hard to do technically and likely to cause breakage and errors. Have you considered reducing the storage or moving other files off the boot volume so your iCloud local cache can be on the same storage as the user folder (and optionally the OS itself?)? That clearly isn't what you hoped, but with this details - maybe you can move all user files or come up with a different accommodation.
If you boil this down to a yes/no question on macOS 10.13, I would say it's not safe to try to move that data elsewhere and you'd want to thin the data entering iCloud of you don't want 76 GB of it on the Mac where your home folder exists.