I have been struggling a while with the intent to run OPNsense 18 natively on the APU2. I have given up on this endevour and am now going after running Debian Stretch on the Hardware and OPNsense 18 as a VM.
I settled for virtio NICs now, as the IOMMU patch has not been merged and I have already compiled my custom BIOS 4.6.6 with console on COM3 which is internally connected to COM4 of the second APU2 in the box and vice versa.
Interestingly enough the virtual OPNsense feels a bit more snappy than the native one.
I am mitigating the reboot issue in 4.6.x BIOS series with the USB-Watchdogs...