I think you are trying to apply a 6-year lookback rule that would include years 2018-2020. The year 2020 will not count since this was the year you were non exempt because of the two-year exemption rule for J1 research scholars.
Now you became a F1 student so your exemption period is 5 calendar years starting in year 2021. Since you have already been exempt two years, you will be exempt in years 2021,2022, and 2023. Next year however, your exemption period will have ended and if you qualify to be a resident under the substantial presence test, you may file a resident return.
Please read the following IRS guideline and look at Example 10. This explains perfectly what i just mentioned.
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