For the moment, until Apple sorts out the bug involving the T2 Security Chip and external USB audio interfaces / core audio (including, but not limited to Mac Mini 2018), you have two options:
1. Buy a Thunderbolt audio interface
2. Use a certified Thunderbolt docking station / hub with integrated USB circuitry that bypasses the Mac's internal USB, like the Caldigit TB3 docks to connect your USB audio interfaces.
These are the ONLY proven methods to bypass this bug. Many folks out there have tried creative workarounds for fixing this, but with only temporary or no success.
Check out this GearSlutz thread (one of many regarding this topic):
GearSlutz USB Audio Thread