I first entered the U.S. in September 2014 on an F-1 visa and have always been an F-1. 2018 was my 5th calendar year as an F-1 and I will meet the substantial presence test in July 2019.
Can I use the First Year Choice to file as a resident alien for 2018, given I have stayed in the U.S. for the entire 2018 calendar year?
No, you will have to file a non-resident return for 2018. A person who is an exempt individual (in your case the F1 visa) is not considered to be present in the United States for purposes of the substantial presence test. In your case, the days don't start counting until 2019. Please see IRS Alien Residency Examples F1 visa for more details. You will have to file form 1040NR and use our partner Sprintax.