It’s always about the economy, you’re right. And it’s the one thing Trump has working in his favor (and it’s a huge one to be sure).
But if the economy starts the really struggle, then where does he go? Not that I want the economy to struggle, but I am really not sure what he could bring to the table.
The democrats have to just be wary of getting sucked into the personal aspects of how Trump campaigns (a lesson I need to take to heart on here) and be firm on their platform. The other democrats who are left also need to fully back the eventual nominee. Punching down to his level won’t work.
But, as you state, the single largest voting issue will always be the economy. Trump is smart enough to do some manipulation into 2020 to keep the economy in his favor. If it's strong, like it is now, Dems will have few talking points to attack him on and most of them are going to be secondary in voters minds to their checkbooks.