I had this same problem. Took me a while to figure out, but it is not a problem with the aggregator or Catalina.
The issue is with your Rekordbox subscription. You need to purchase a core or premium subscription as your CDJ350s do not qualify as "hardware unlock" for Performance mode. The subscriptions are currently discounted and also has a 30 day free trial.
Once you have signed up for either Core or Premium, you need to go into Rekordbox 6, click the "MY PAGE" button (next to the "settings" cog at the top of the screen, then click on the "PLAN" tab. Your plan will be shown here, you need to click the "Activate This Computer" button.
Once activated, you will be able to select your deck on your CDJ350s and then use them as controllers.
I hope this helps you.