In the end, all any one citizen can do is vote their conscience, and do so while being properly informed. The properly informed citizen knew exactly who Trump was all along, and as any responsible adult, any responsible human, any true American citizen, voted against the asshole. Any adult knew and did everything they could: voted against that fuckstick.
It is no other way.
The problem, as you suggest, is that such of us incorrectly assumed that there were as many adults in this country that understood just as much, that were responsible enough to inform themselves on the issues, shuffle through the obvious bullshit, and vote accordingly. Yes, many were wrong to trust that their neighbors were responsible American citizens, but what would have done then to prevent what happened? Suspend the law, shut down Fox news and the other ultra-fundy rightwing duplici-sphere outlets, arrest the Trumps for their then-known relationships with foreign antagonists, and force the GOP to select a mature human as candidate? I don't see any other option to make things right, because their mob had long-been gaslighted into subservience at that point.
Since we're in time-travel mode, do we just back to 1964, ensure that Ford tosses Nixon into prison, where he belonged, so that future parties actually learn that there exist real consequences for true crimes against this country and its people? DO we lop off the republican head that ushered in the fundy evangelical nutbags into the center of the party, against all desires of the real intellectual center of the GOP of that time? There are many points at which the GOP has failed this country, again and again and again that inevitably lead to Trump, yes--but at the end of the day all anyone could have done, really, is expect our fellow citizens to think and vote like cognitively-mature adults. We now know that this is not possible, as many are held in thrall by the phantom promises of the rich giving you all their money--if only we charge them less!--of education magically improving, if only we fund it less!--of the climate magically fixing itself, if only we ignore that we are fundamentally doing horrible things to destroy it! etc etc.
The only real disease in this country is the GOP and their morally, economically, philosophically, bankrupt platform. Remove this minority party from power, and get the country back on the track it deserves to be on.