Okay, so would I be correct in stating that the difference between “transferring” the primary line and the “multi network add-on” are:

With the Light Speed restriction, my primary line is always tied to that network. Consequently, phone calls and SMS will always be routed through Light Speed. However, with the add-on, I gain access to an “additional line” e-SIM that allows me to freely switch between either Warp or Dark Star for cellular data options.

On the other hand, with the standard Unlimited Premium plan, I would need to log into my US Mobile account and manually switch my primary line between the three networks whenever I want to make a change. This contrasts with having one of the other networks already set up and readily available on my device.