Having come to the USA on F-1 in 2015, means that you would have been an exempt person till end of 2019, were it not for change of status in 2018 Oct. So your test period starts on date of adjustment.
Thus you would have had to meet the following to be able to exercise first year choice --
(a) you were NOT a resident in 2017,
(b) had been continuously present in USA for at least 31 days in 2018
(c) during the test period you were present in the USA for 75% of the days --- so depending on when you left the USA in Dec 2018 , and how long you stayed away, you may or may not have met the 75 % -- there is also a grace period of not counting up to 10 days absence.
(d) you would meet the substantial presence test in 2019
So we need more info to comment on this first year choice.
Assuming that you had met the requirements and you were able to make the first year choice, you still would run into the issue of filing joint because your spouse had not been admitted to the USA. If she had entered and then went back home and did not come back till 2019, you could still file jointly, with the form W-7 filled out.
My suggestion would be to keep it simple and just file as Non-Resident Alien using form 1040-NR ( TurboTax does not support it -- consult SprinTax, Tax professional or your school foreign student's office ).
Namaste ji
