Instead of E-2186G, I just bought an E-2176G from a cybershop in New Zealand for better power savings.
And my build is based on Gigabyte MX32-4L0 and 4x16G ECC Crucial CT16G4WFD8266.
Both unraid and ESXi 6.5 work like a charm. Both RX590 and 1660Ti could be passed-through to VMs without any issue.
If I want to go CoffeeLake and ECC with a relative high frequency, I think E-2186G or E-2176 is the (only) best choice at this moment.