The 6 year look back rule applies when you first enter with a F-1 visa and then return the US with a J-1 visa. Not the the other way around.
You are a resident in 2020. When you first entered with the J-visa then F, you fall into the five-year life time exempt category for the F-1 visa. So, you count 2012 ( while you were on J-visa), 2016, 2017, 2018 and 2019 for the years that you do not count days. Starting from January 1, 2020, if you stayed in the US for more than 183 days, you are a US resident.
@MightyOwl326
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