Hmm, so looks like ZFS did end up using all that available disk space over time and it doesn’t know how to punch holes back into the sparse file, so it can grow but never shrink.
Your options at this point are somewhat limited since you can’t shrink a zpool.
It may be easiest to define a new ZFS storage pool in LXD and then move your containers over to that using the --storage option of lxc move. Once all the containers have been moved, you can update your default profile with lxc profile edit default and then delete the old storage pool with lxc storage delete.