Qemu supports more CPU models than listed in Proxmox. Updating the models in /usr/share/pve-manager/js/pvemanagerlib.js seems to be enough.
kvm -cpu help
Available CPUs:
x86 486
x86 Broadwell-IBRS Intel Core Processor (Broadwell, IBRS)
x86 Broadwell-noTSX-IBRS Intel Core...