Is it far better than DEPNOTIFY and SwiftDialog for mac device onboarding?

This is entirely in swiftDialog and replicates most of the functionality of DEPNOTIFY without requiring you push an additional package to the machine.  It also allows more choices both on registering the computer and what all gets installed.

So where is this package SYM-HELPER and where I need to install it, how it will help me?

SYM-Helper, as the blogpost linked above explicitly explains, is a tool you use on your machine to generate the Setup Your Mac script.  It uses the API to pull in buildings, departments, and policies to make it much easier for you to customize the SYM script for your environment.

It provides the end user with much more information while on-boarding as well as allowing for much more user input all packaged in an easy to understand gui.

It depends on what you need to do during onboarding, I wouldn't say it's better or worse, it's a new tool for specific usecases but overall, depNotify hasn't been updated in years so this is something we can count on moving forward.