I think a certain amount of pessimism is understandable. The vast majority of republicans are ethically challenged and would have no problem 'cheating' to win. (Attempts at voter suppression, smear campaigns, gerrymandering, etc.)
But, I don't think there is much to worry about if the Democrats pull off a clear majority in the house or senate. Yeah, republicans may try to launch a bunch of court cases, but so far judges/juries have been one of the few functioning guard rails of US democracy. (Even as pro-republican as the supreme court has been lately, they don't always side with all of Trump's shennanigans.) What else can they do? Arrest successful candidates? Those cases will fall apart quickly. Not have them sworn in? From what I understand it doesn't necessarily have to be the vice president or speaker who swears them in, it can also be a sitting judge.
A more likely scenario is that Democrats will be seated in congress, and then Trump will just continue to act unilaterally (as he has been doing much of the time anyways), ignoring congress.