I think it is better to go from the 2018 Cruz- O'Rourke difference of less than 215,000 votes with 8.3 million voters. Although in 2016 3.6% voted Libertarian or McMullin, so Republican never Trumpers will throw away ~300,000 votes in 2020 too.
I also think that turnout could vary widely depending on who is the Democratic candidate. for Bloomberg and Biden it would be closer to 10 million, while for Bernie greater than 10.5 million. That could easily be the difference. The key to winning in Texas is not turning swing voters, it is getting traditional non-voters to vote.
~85% of the 2 to 2.5 million voters who did not vote in 2018 will come from the 10 largest counties, which lean democratic. I think the margin in the 10 largest counties will easily be over 215,000 votes greater than in 2018, before factoring in the Libertarian vote. Republicans with have to work hard at suppressing the democratic vote while registering more Republicans.
I think that Trump beats Bloomberg or Biden in Texas, but loses to Bernie's turnout.