I think you make a really excellent point about Trump commencing his run so obscenely early for no political benefit but instead because of his legal issues -- he's worn out his base and maybe even himself with the incessant apocalyptic rhetoric and fund-raising. There were more than a few GOP strategists in 2022 and 2023 warning that Trump was abusing his grass roots fundraising to pay for his legal fees in ways that would undermine his calls for political fund-raising in 2024. But maybe the negative impact is more pronounced in his messaging than his fund-raising.
Maybe Trump Fatigue is finally catching up ... Barrack Obama had a great line last night comparing Trump to a neighbor using his gas-powered leaf-blower outside your window 24/7. I think that a good portion of the unexpected eruption of support for Harris is not just a celebration of "finally, they listened to us about not wanting an 81-year-old candidate" but also "finally a fresh message and messenger to counter the grumpy old men calling each other Nazis and communists."
Frank Luntz has been very critical of Biden's dark rhetoric about Trump as being too over the top, too dark and clearly turning off voters in Luntz's focus groups (basically saying nobody seems to believe Trump is the next Hitler, even if they loathe Trump). Luntz has the same critique about Trump's communist! Red Scare messaging about Harris -- sure, MAGA trolls seem to enjoy it, but ordinary voters see it for the bullshit that it is and are sick of the extreme rhetoric. Luntz is out there every day begging Team Trump to drop the Red Scare and DEI and sexist attacks and just hammer a few things, mainly inflation (not the economy generally, just PRICES people see in their local grocery store and restaurant) and immigration.