_SMSTSReserved variable: hide password?

This really has nothing to do with UI++, this is simply how the task sequence stores this account and its password and it can be retrieved in many other ways. Hiding it in UI++ is a good idea but keep in mind that this won't prevent these values from being retrieved in other ways. This is why its imperative that you use a least privilege account for your network access account. Also note that the network access account is not a service account, it's simply an account used for fallback authentication to DPs.