If you long click the media button on bottom, you can drag it to the far left side. Whatever is on the left is default. Even after doing do, mine still wants to default to USB even when my phone is currently connected and playing though, if anyone has a solution for that let me know.
You can long click the media button on the bottom of the screen, this allows you to move the media option to the far left on bottom. Whatever is on the left is what it try’s to default to. I am having issues with mine defaulting to Bluetooth within media though, always seems to want to default...