Using Hyper-V with USB3 Ethernet

I've currently got an ER-X running Openwrt and all is good.
I'd like to setup a backup option in case the ER-X dies. I have an old Intel NUC i3 with Windows 10 Pro on it and Hyper-v.
My DSL is only 80Meg down/15meg up.
If I get a couple of 'cheap' USB3 Ethernet would I be able to get Openwrt running , or am I heading for a world of hurt?

I was able to make it run as a Hyper-V VM and as all of network adapters should be presented as virtual, this should work at least as backup.
I don´t have USB NIC here to test, but I don't see a reason for hyper-v not recognize and allow them to add as virtual network.
Install was really simple.
Create a gen1 VM, attach vhd and any linux live cd.
Boot from this live cd.
On terminal:
gunzip openwrt-19.07.7-x86-64-combined-squashfs.img.gz
sudo dd if=openwrt-19.07.7-x86-64-combined-squashfs.img.gz of=/dev/sda
Eject cd and reboot VM
Atention: Make sure that default IP address (192.168.1.) will not conflict with your network. Also disable DHCP 4 & 6 to not cause havoc to your network.

If I read this correctly, you're willing to buy a USB 3.0 ethernet card for this backup scenario, which costs roughly 20 EUR/ USD (you need at least one)…

You already can get a cheap ath79 single-band device for that budget - twice that (quoting you as "a couple") would get you a quite usable concurrent dual-band router as well. While I wouldn't quite recommend devices from this (very initial-) entry level class, any recent ath79/ mt7621 device can cope with <100 MBit/s easily and can act as a temporary backup device (which doesn't necessarily needs meet all your usual amenities).

If you include the used markets into your selection and add some patience on top, you can get quite amazing devices for very little money (10-15 EUR/ USD if you're lucky, 25-40 EUR/ USD if you're less patient).

Thanks, a friend has some a couple of spare usb to ethernet dongles I can use.
I'll give it a go.

I agree I could get a cheap backup device. 2nd hand Er-x is ~£30 off ebay.
I was doing it for the technical challenge as well.

There isn't really a technical challenge (OpenWrt works fine on x86_64), the real challenge would be to have everything in functional condition when you need it most.

On a plastic router you have sitting prepared on your shelf this tends to work just fine, on an x86_64 that also sees other uses (and is not dedicated to this one task) the situation is different, and the wish to run OpenWrt under virtualization complicates the situation even further. Using virtualization, the host has to cooperate, passing through the network cards (that's not working too well with USB, IOMMU support on the CPU and PCI(e) has a huge advantage in regards to performance and reliability here)), you basically have to do (essential) parts of your networking configuration twice, once on the underlying host (so the necessary ethernet links are isolated from the host and correctly passed through to the VM) and another time on the OpenWrt VM - this is fragile and breaks easily, especially upon changes (upgrades on the host or upgrades for the OpenWrt VM), which are easily missed on 'spare' devices you won't test regularly - with the expected results whenever you really need it 'now'. A cheap/ old plastic router that (at least-) barely keeps up with current OpenWrt is more likely to 'just work' in such a situation (the hardware is known, all drivers preinstalled and basics pre-configured, there is no 'unknown' virtualization underneath).

There are multiple uses for virtualized OpenWrt systems, but I wouldn't quite recommend it as backup.

if you buy USB ethernet adapter i'd recommend to buy the ones using ASIX USB-ethernet chipsets and not Realtek ones.

While Realtek usually work they can lock up if put under load and may or may not come with the same mac address which will confuse your devices (either on the host side or network).

You can just search for "ASIX usb ethernet" in Amazon for example to get devices using these chipsets.