one point to remember for the additional script, that it needs to be run after the DataOnly Upgrade completed, menas at the end of whole upgrade process.
now comming to your below questions
" If I move System setting to DSS, then I need to change the calling function also. This will increase Post upgrade task. Please correct me if my assumption is wrong."
it is basically if you have any Pega specific system stennings you have changed in your old prconfig.xml those need to be converted to DASS (Data Admin System Settings), even that step is not mandatory but suggested.
also not sure why you are asking whether you need to change the calling functions also, because these changes are specific to which are Pega PRPC specific setting (admin stuff related, etc), why you need to change your code?