I agree that the Civil Rights Act was a turning point, but the GOP (i.e., Nixon) could have chosen not to adopt the racist vote. (How do you leave RMN out of your list of bad guys?) Goldwater seems to me more complicated than his "no" vote on the Civil Rights Act. See the Wikipedia article on his life, which includes a section about his Civil Rights record. I'd take him off the list and put Nixon on.

I don't want to conflate conservatism with racism or obstruction. https://remarklj.medium.com/the-politics-of-obstruction-12a0fdac17e9 Nixon nursed the racist snake back to health, and Gingrich and, especially, McConnell, broke the government. https://remarklj.medium.com/mitch-mcconnells-moral-luck-a847c7cca13a In that sense, Trump was certainly more symptom than disease. Still, I think the party "died" fifty years to the day after August 6th, 1965, but you have indeed identified the date on which it encountered the racist carcinogen that ultimately killed it.