At the moment if you first attach a storage pool to the existing lvm thinpool and try to delete it then it tries to delete it as well:
$ lxc storage delete default
Error: Failed to run: lvremove -f /dev/vghdd/lvdata: Logical volume vghdd/<redacted> contains a filesystem in use.
How about a parameter that simply removes a reference to the thinpool and leaves it as-is?
PS: I could have tried to contribute it, since it’s obviously has the lowest ever priority, if needed at all