Substantial Presence Test, Calculation

Hi all,

 

I'm an F1 student and first arrived in 2015. I was exempt from the substantial presence test in 2015, 2016, 2017, 2018, 2019. I began counting days toward the test beginning 01/01/2020.

 

So in 2020, I become a resident for tax puposes (183 days on 07/01/2020). I filed 1040 as a resident in April 2020 for tax year 2019. Is my thinking correct? Or I can only file 1040 in 2021?

 

Thanks!