I gather from the Proxmox docs and other sources that the OVF doesn't have a standard way of setting the VM to use UEFI instead of BIOS. Does Proxmox support any of the common (VirtualBox / VMWare etc) proprietary ways of setting UEFI mode?
If you are importing an OS that requires UEFI it should be set by default if you select the correct OS in the Import-GUI. For example if you choose Windows 11.