Have been eyeing TR for that reason, but as I don't really need this amount of I/O (and cores), I might be well served by AM4.
CPU: AMD Ryzen 7 1700
Mainboard: GA-AX370-Gaming K5 (Bios: F25)
GPU (Host): Gigabyte GeForce GTX 970
GPU (Guest): Gigabyte GeForce RTX 2060
OS (Host): Arch Linux
OS (Guest): Win 10
As said it appears to me IOMMU grouping depends a lot on the board used. For mine it looks like all the chipset external PCIes are in one group and the CPU PCIes (basically the 16x for GPUs) are in two seperat groups. So I run two GPUs on 8x on each slot now. I was also able to pass through an USB 3.0 on-board controller as it was also in it's own group.
But there is the ACS patch so you are able to pass through single devices which are in a physical IOMMU group (not only cpu lanes). I don't have personal exp with it tho.
Warning: There are issues with the latest Ryzen gen and bios updates and they break vifo.
In addition to that I use the VM mostly for fullscreen application, either games or some windows only software so I don't get any advantage out of the windowing.
Instead I decided to take an auto-switching approach via CDD and some shortcut menus to switch GPU outputs on my main display, see in action here: https://gfycat.com/tepidliquidcero
USB passthrough with an Startech USB PCI-E card was a bit problematic as the group was not isolated. Turned out the 2080 has a USB controller for the type C (I'm told for VR).