I realize that. I still can't wrap my head around that. We already know what convict Trump is. We also know what Biden is. Neither are perfect, but only one is an existential threat to this nation. The base of convict Trump will never see that, but the independents should (IMO) see the red flags.

No, independents don't and won't see any red flags. Trump was already President once, and while there were plenty of things that happened during his Presidency (both good and bad), the country survived and continued to thrive. For most people (who don't pay too much attention to politics), that's a huge counterpoint to Democratic assertions that Trump is an existential threat.

Had Trump been found to have played a more direct role in the riot of January 6, rather than having only given the speech and not doing much to stopping it, it might have been a different story. Instead, though, the story of the first Trump Administration is one that ended up being one where some conservative/populist policies got implemented, most did not (no repeal of Obamacare, no ravaging of the social safety net, no drastic changes to most laws), and you had the expected appointment of lots of conservative judges.

Against that, Democratic fulminations about an existential threat are simply not landing. And they're not going to land any better in the next four months than they have the last four months. The voters already have a really strong sense of who Donald Trump is, and believe they have a good sense of what his Presidency would be like (since they already went through one term), and have mostly made up their minds about it. Democrats are very unhappy with the conclusions that the voters have drawn about Trump, but he's perhaps the most well-defined and completely-vetted Presidential challenger in modern election history. So there's not a whole lot that can be done about it.