First and foremost, there is no guarantee with PCIe pass through so take ALL recommendations with a grain of salt.
I generally recommend Gigabyte motherboards to new Unraid users for one feature that is the ability to pick which PCIe x16 slot to boot with. It's called "Initial Display Output" in the BIOS.
That would simplify things when you need to dump vbios i.e. no need to physically swap GPU. Boot with slot 1 -> dump vbios for slot 2 -> reboot to BIOS to boot with slot 2 -> dump vbios for slot 1.
It also gives you flexibility of GPU placement e.g. if card too long, too wide, too big etc. The RX 570 is one of the known bad child of reset issue so the ability to NOT boot with it even if it's on the 1st PCIe slot is priceless.
On a side note, for Ryzen and 2x GPU, make sure you get a mobo with 3x PCIe x16 slots to give you some future flexibility too.
And for the 2080 Super, it has 4 devices (GPU + HDMI Audio + 2 USB devices) that have to be passed through together - other graphic cards usually only have 2 devices. This tends to catch new users off-guard.
You need to stub the USB devices for them to show up on the VM template (the easiest method is to install VFIO-PCI Config plugin and use it to select the devices and then build VFIO-PCI.CFG file then reboot).
Good luck.