Here's a timeline of when I believe each candidate was probably favored during Trump's presidency based on the results.
January-May 2017. Trump would've won.
June 2017-April 2019: Biden would've won. (Maybe Trump would've won at a few points in mid-2018)
May 2019-April 2020: Trump would've won.
May 2020-present: Biden would've won with perhaps a few days in mid-late August where Trump would've won.
Why do you think odds would have flipped in Trump's favor in Spring 2019 until after COVID hit the next year? His approval ratings never really moved much. Also why is mid-late August 2020 the exception for Trump? His polling was abysmal throughout most of the summer. Is it just the convention? Because there wasn't really any bounce.
Personally I think Trump
could have won pre-COVID (and maybe even after if he did things very differently), but by no means was it ever guaranteed or even was he necessarily favored. He was a VERY polarizing figure, with people very strongly divided into two camps about him, but always more against than for. The approval ratings were very consistent about that, with virtually nothing ever moving them significantly in one direction or the other. Because of this, I'm not honestly certain that at any point the outcome ever could have been
drastically different. I think a state or two could have flipped one way or the other but that's about it. The polarization is just too strong right now.
Hell, in my view he deserved to lose in an epic landslide more than any other incumbent ever, Hoover included, but I'm not sure such a thing is possible under any circumstances at the moment. If you're even right about just FL/NC/TX at most flipping even after 1/6, which I suspect you are, that's a pretty telling sign of just how deeply entrenched we are as a nation into two diamatretically opposed realities right now.