For Trump to nearly maintain his vote share in Texas while dramatically lower percentage of people voted 3rd party would have taken ~100% of third party defectors voting Biden. That’s simply unrealistic.
Your nationwide numbers are also off for several reasons. For example many people voting in 2016 skipped the 2020 elections for various reasons like death. Further pooling including exit polls doesn’t match up with actual voting very well suggesting a significant portion of Trump voters are unwilling to say they voted or plan to vote for Trump. That or a truly massive pro Republican voter fraud which seems unlikely.
Anyway looking at hard data. In 2016 Clinton got 65,853,514 votes Trump got 62,984,828 votes. A Democratic margin of 2,868,686 million votes. In 2020 Biden got 81,282,896 votes vs Trumps 74,222,484 a Democratic margin of 7,060,412.
Using the middle of your range 62,984,828 * 0.075 - 65,853,514 * 0.035 that’s a net gain of 2,418,989 votes which far less than the actual net gain of (7,060,412 - 2,868,686) = 4,191,726 votes that Biden over Trump.
To gain the (4,191,726 - 2,418,989) = 1,772,737 votes from a 3 million 3rd party voter drop would have taken a truly extreme margin of ~80% even assuming your other assumptions where correct. That might be reasonable if they where mostly green voters, but it’s largely from the Libertarian party.