SCNLookAtConstraint, specify how the node looks at the target node?

The documentation for SCNLookAtConstraint notes:

When SceneKit evaluates a look-at constraint, it updates the constrained node's transform property so that the node's negative z-axis points toward the constraint's target node.

But SCNText geometry is created facing forward along its positive z-axis, so your constraint system needs to account for that in some way. I advise using SCNBillboardConstraint, which is designed for this kind of case. Its documentation says:

An SCNBillboardConstraint object automatically adjusts a node's orientation so that its local z-axis always points toward the pointOfView node currently being used to render the scene.

With this, you shouldn't need to reparent the node.

The "SCNBillboardConstraint object automatically adjusts a node's orientation so that its local z-axis always points toward the pointOfView node currently being used to render the scene."

textNode.constraints = [SCNBillboardConstraint()]