I fixed it by using the steps above:
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1) sudo systemctl stop apache2
2) sudo mv /var/lib/svn/* /home/user/data
3) Fstab: UUID=fb4a08b7-378e-42eb-9b7a-2c7b4f85cd06 /var/lib/svn ext4
nodev,nosuid 0 2
4) sudo systemctl start apache2
Now the svn dir and below are on the other partition and the system behaves
normally with an 18GB margin for / being full.