I attempted to work around this issue as suggested with a bind mount line in /etc/fstab linking /home/user to a directory on a separate ext4 partition that is itself mounted at /mnt/files:
Here are lines from fstab:
UUID=465bfd74-ce84-4fbe-9af4-8b2d88768352 /mnt/files ext4 defaults
/mnt/files/bionic/home/user /home/user none bind
I discovered that this stops Gnome trash from being able to move any files to trash in directories on /mnt/files (Ubuntu 18.04). It will present a dialog saying the file cannot be put in the rubbish bin, and instead must be deleted immediately.
Testing with gio shows why:
gio trash /mnt/files/testfile
gio: file:///mnt/files/testfile: Unable to delete file /mnt/files/testfile across filesystem boundaries
Any ideas on how to fix?