The biggest mistake that leftists make (and are continuing to make) is to separate cultural and economic voting motivations. It's true that they can be distinguished, but they are deeply interlinked: the state of the economy & the experience of economy are not the same.
The racism that Trump has espoused has been successful because of the deep economic precarity present in the US. It's no surprise that the biggest issue that voters cited as the motivation for their vote choices was the state of the economy.
It's hard to argue that Biden reflects any departure from the economic policies and approach to governance that got America to where it was in 2016. Regardless of what you think of him, it's hard to argue that Trump is a continuation of that period - from how he presents himself.
A lot of his actual policies are in fact very much a continuation of Republican norms (with some added racism), but ordinary voters are far more aware of rhetoric than the policy that impacts them. The consequence is that they care more about what Trump says than what he does.
Florida voted against Biden, but voted to increase the minimum wage - that contradiction is an important one that speaks to my point. Democrats need to both speak to and address this economic anxiety. Their current language is totally failing at that.
The reason people constantly say that Bernie would have been better as a candidate is because he is one of the few Democrats who does this. He doesn't try and 'thread the needle' of talking change but keeping things the same, he positions himself as changing the status quo.
The racist animus in the US has been fuelled by economic hardship. It's a deeply economically unequal country, but a lot of the rhetoric in US politics is about addressing racial (not economic) inequality. For a lot of people that sounds like their inequality doesn't matter.
Hearing other forms of inequality given primacy transforms the experience into a cultural one. You can't ignore the economic foundations of that cultural experience.
Just to be clear, I think that racial equality and racial justice matter deeply - but you have to understand how the way we talk about these things can put offside some people who continue to be economically precarious.
And yes, many Trump voters aren't poor. It doesn't mean that the foundations of his movement are not driven by a failing economic system that is driving increased economic inequality.