I have not upgraded to Catalina yet, but I just checked DJI Assistant 2 for Mavic v2.0.8 and it is a 64-bit app.
The last time I tried using Assistant, I thought there was an issue as well connecting to the Drone but it turns out I was using a USB-C cable that only supported charging (ie no data transfers). Perhaps that is your issue? Even if the cable you are using is supposed to support data + charging try using a different cable. Good luck!