I have a treeiter created with the treeiter = self.devices_treestore.append(parent_treeiter, column_values_list) call.
How can I move it to another parent (with the whole subtree it holds)?
Assuming the new parent is at the same level of the tree, you could use treestore.swap, otherwise you may have to just remove all the rows in the subtree and reinsert them at the new position
GTK+: Move tree element to another parent