Can I create another dummy tax return with different input to better determine what my estimated tax payments should be for 2019. How do I do that?

You would have to use a new online account.  Or use the Desktop program.  You don't have to pay unless you want to print it.

Online is only good for 1 return.  But you can use the same email address for 5 accounts.   How to start another return in the Online version https://ttlc.intuit.com/replies/3288574