I think about move /tank1b/subvol-307-disk-1 /rpool/subvol-307-disk-1.
both storages are zfs.
GUI is not allow it.
so, backup that container, delete and restore for taget storage, move success.
Do a zfs send/recv of that dataset from rpool to tank1b (google how to do it) and then rename the data source in container's config file (/etc/pve/lxc/307.conf)