move boot partition of proxmox (or debian) to USB flash, installed on two NVME disks in zfs-raid-1 on a non-bootable legacy bios

I am trying to setup proxmox on Dell r720xd, which does have UEFI boot option but can not boot from PCIe nvme drives. If I install proxmox (or any os) on a usb flash, it boots fine. For testing, I installed CentOS with advanced partitioning, and used USB flash as /boot partition, and nvme for all other partitions and that too works flawlessly. With proxmox, there is no custom partitioning option during the setup, so I was unable to set usb key as /boot partition during install. Other option listed in proxmox forums are - use clover as bootloader on a USB key, described here - install debian first with custom partition and then install proxmox over it and remove debian kernel. Obviously clover is adding a boot partition on usb and then forwarding the control to nvme. My question is, how to install proxmox on nvme and then migrate the /boot partition to the usb flash? Seems like a very straight forward process and I followed this guide to handle it, however, I could not get it to work. grub2 always complained with messages like - not matching UUID - you need to load kernel first - and so on any hints please. P.S. I also tried to edit the centos /boot installed on flash drive, by using grub2-mkconfig which found the proxmox and updated the grub.cfg but still got errors during boot like - error: cant't find command linux - error: cant't find command initrd fixed these errors (ref ) by linuxefi and initrdefi but then it could not find the drive with given UUID