The Pubs' message is nationalized. "Supreme Court, Pelosi, George Soros, MS-13, grwahrrr!" This is great for them for the highest level races (Senate) but declines precipitously in effectiveness as you go down-ballot. Even at the Congressional level, more targeted approaches that factor in local politics should hopefully get a lot of Dems elected. Let's cross our fingers that it also pays dividends further down ballot in state and local-level races.
I agree with your scenario, but I don't think this will be bad for Trump. Evidence points to Trump doing better in the polls the more he can stay out of the news. The Kavanaugh hearing and passage of the tax cuts are really the only events of his presidency where he saw a bump in the polls during and following their occurrence. Everything else that has come in the news cycle has hurt him. Independents are driving his approval rating now that the D's and R's are all dug in.
I'll go a step further. If his shit-show rallies become passe, they will be yet another of his worst traits to be normalized within American politics. That's bad for us all. If he gets reelected, this and the larger subject of voter fatigue would have been a big reason why, despite any problems it causes with Republicans down-ballot.