Can I file now with an estimated income and refile later once I have my W-2? I am still waiting on my W-2 from my employer but I need to file to sponsor an immigrant.

File what?  For what tax year?  How could you still be waiting for a 2018 W-2?  They were issued in January 2019 for 2018 tax returns.  What are you trying to do?  You cannot file a 2019 tax return if that is what you are asking about.