Fortunately for the Democrats, Sanders learned his lesson by 2016 and fully supported the Biden/Booker ticket. This wasn't enough to overcome the Republicans, but at least the Democrats managed to hold their own in what was overall a good year for the GOP.
I was surprised at how there was no net gain for either side in the Senate. Sure, Joe Heck won in Nevada, but it was cancelled out by Tammy Duckworth’s win in Illinois. And, Republicans only gained 7 seats in then House.
I was surprised by Booker as Biden’s VP pick. He’s from a state that is solidly Democratic at the national level. And he was only in the Senate for two and a half years before becoming Biden’s VP pick.