Just to close the loop for anybody who stumbles upon this problem. Thanks to new member brocski, updating to firmware 5516 solved the issue for the both of us. My phone is a Pixel 3XL and his is a Pixel 3.
Finally got approved to post on here! Glad this worked out well for both of us. I tried 4 different cables and was about to throw in the towel before updating the firmware. The head unit is 100x faster than it used to be. Stoked on the fix!
Also, this is a pretty handy trick to avoid plugging a cord into the front of your dash. I soldered a VW USB cable to the back of the circuit board of the RCD330 to relocate the USB to a button above the shifter. Gotta make sure the wires are on the right contacts but otherwise it's pretty straightforward. I think there are two variations of the cable out there so be sure to figure out which one you're working with before soldering.
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