He should have talked a lot more about the economy, that was his best issue. He didn't really have a coherent message for 2020, not like 2016, even something as simple as reminding people a lot more 'The economy has been great under me, and I'll keep it that way'. If he'd done that, I think he would have won.
Would this really have worked that much, though? For me it seems that it would be a bit tone-deaf to go on and on about how great the economy was when there was mass unemployment, in a crisis that he himself mismanaged.