I used an AMD e-GPU and two 4k LG monitors with the Mac Mini 2018, the system is Mac Catalina 10.15.3. The Cubase always make the whole system stopped and I need to force the mac restart with the power button. The Cubase can run normally Only if I set up the Cubase to " Open in Low Resolution" in the “Info Menu”. 
I dont think this is a cubase issue. If the hole system hangs it’s most likely a hardware or OS issue. With cubase 10.5.10 cubase pushes the GPU a lot harder, that make that the GPU need’s more power and more cooling. Can you run geekbench5 graphics tests without problem?