This paragraph (that immediately followed some you highlighted that I think were really good) didn't quite track with me:
First, I don't think the section I bolded is completely true. I reside in a place that went heavily for Trump in 2016 and interact daily with what I consider to be his key constituency. I'd argue that the message has not sunk as to the depth or breadth of this administration's corruption and toxicity. They aren't immune to fact either, they just exist in the same media vacuum that the author warns of in point 3 and/or they don't even perk their heads up until just before the election. Campaign messaging, particularly adverts, are one of the only ways to pierce through that bubble (even if the impact isn't huge). You may not be able to persuade a lot of these voters to switch sides but you might convince a few that they shouldn't pull the lever for Trump. As EA has suggested I also think there are large openings around his fitness office that are conversations that need to be had.Trump has dominated the national conversation for 5 years. His flaws are known by all. The Democrats should want this to be a choice between competing, starkly different visions for the country. If it’s all about Trump, Democrats will lose.
Second, I think the author may be overselling the influence the future Democrat candidate will have on steering the conversation away from Trump. in my opinion, the media is the primary problem on this topic and they don't seem to have learned any lessons. Whether it is just for the ratings boost or some sick fascination with the train wreck, the major outlets just can't seem to help but make it "all about Trump". I hope whoever the nominee is can break that cycle but I see no indication from the media that they plan to change their playbook following 2016. I think you can look the way most of the big players framed the Ukraine discussion (particularly early on) to still include some ominous allusions to the Biden's hand in the matter even after the conspiracy theory around them had been thoroughly shredded.
One of the points I haven't seen brought up that I actually thought the best was the OP's 2nd point about framing the economic discussion in a different way. I may be heavily biased by my anecdotal experience on the topic but I've found that by getting people to examine their own personal economic situation vs the Economy-at-large you can make some serious dents in that "positive" for Trump.
My own father is a pretty solid example. He is an aging white guy from the South whose financial solvency was heavily impacted by the Great Recession and who never really got back on top during the recovery years. Although that was due primarily to his lack of secondary education and being in a field that was moving at light speed to greater technological solutions he just didn't understand, it didn't stop him from pinning the blame on Obama. Trump was supposed to be the savior in this regard but so far hasn't delivered much. His wages are still stagnant, he isn't invested enough for this ginned up market to be any benefit to him and he actually saw his tax bill go up under the latest changes to the law. Trump is more vulnerable on this topic than you would think at first glance and I think any Democrat candidate that can both hammer home his unfitness for the office and pair that with a solid plan that resonates with people like my father has a solid chance even given all the incumbent advantages.