I'd say it was around 2014/2015, when there was a huge inordinate amount of focus on police brutality, and when "wokeism" started to become the prevailing cultural ideology on the left, promoted by none other than Obama himself. He relentlessly promoted the idea that women were earning 70 cents on the dollar of every man (not true when controlled) and that black men are being discriminated against by police (true, but divisive). Remember that Obama also promoted the TPP that even most of his party was opposed to, and Hillary was waffly on. That probably had an effect on this demographic that was unfavorable toward international trade deals. People forget but Obama's approval throughout most of his 2nd term was pretty awful, only in 2016 did it imrpove, probably because most of the political discussion was off of him and on Trump and Hillary. A lot of working class areas were being won by Republicans in 2014, whether it was Elise Stefanik's northern NY district, ME's 2nd district with Poliqun, or Iowa's 1st district. That was a sign, even as Republicans still did way better in the college-educated suburban districts in 2014. But 2016 really was a dramatic shift in the political coalitions that not a lot of people could predict the extent of.
You have brought up a lot of great points. It definitely does seem that 2014 was a real turning point politically and culturally. Just a few observations about that year that I want to ad.
1. With the rise of ISIS and the surge of minors crossing the Rio Grande that summer, Obama and the Democrats came across being negligent and in denial about those situations. In 2016, Trump's two biggest issues were Immigration and Terrorism. While a lot of voters support ending endless wars, when those countries collapse (or nearly do) without our presence, public opinion really shifts. I can remember people not wanting us to intervene in Syria in 2013 only become major hawks on ISIS two years later.
2. Hillary had her book tour, which did not go well and demonstrated that she had a lot of work to do to be a viable candidate for the Presidency. Also Bill Clinton hit the campaign trail for a lot of Democrats in 2014 and had little effect on preventing the loses that the Democrats suffered that year.
3. I remember observing a lot of Republicans and Conservatives stating that they felt that the Republican party was not representing them and that they were allowing Obama to do what ever he wanted. While I have issues with this belief, I do believe that 2014 was year when the disconnect between the Republican Party and its voters reached a critical mass.