Update: After wiping my drive and replacing my old dual boot setup with a fresh install of Endeavour, I tried again to install Windows 11 via virt-manager today and it worked.
I have no idea if it was something in my old installation getting in the way or whether there was an update that fixed the issue for QEMU/KVM but at least its working with virt-manager now!
In case it helps anyone in the future, here are the settings I used for the Windows 11 guest after installing edk2-ovmf and swtpm as per this guide:
Firmware: UEFI x86_64: /usr/share/edk2/x64/OVMF_CODE.secboot.fd
TPM: Emulated | Hypervisor default
Display: Spice
Guest tools: I used the latest virtio-win-xxx-.iso from here to install the display driver via Device Manager (select the default Display Adapter > right click ‘Update Driver’ and have Windows search the iso in D:) and then running the virtio-win-guest-tools.exe for the other drivers.