I've been forwarding an AMD GPU to both Windows and macOS for literally years across multiple Asus motherboards and, except for the specific problem of host sleep/wake, had no problems whatsoever, even considering I work under GPU-passthrough VMs whole-day, every-day. Perhaps try a recent Asus motherboard?
> have to dual-boot QEMU VMs or disable the passthrough
Yes, you would have to buy as many cheap, secondary GPUs as the number of virtual machines that you want to run in parallel.
> I really want proper GPU virtualisation...
Sure, I don't blame you - my point was that the only truly usable GPU virtualization solution available today is GPU passthrough and that GPU passthrough is much easier to setup than it is commonly perceived.