Boot from PCIE device through Harddrive

I have two M.2 drives in PCI adapters connected. I want to install FreeBSD on the M.2 drives connected to the PCIE slots but the trick is the BIOS doesn't support booting from PCIE devices.

I thought I could use the manual partitioner to put the EFI and boot partitions on a harddrive but it gave me errors. Something about not finding a root FS. I wanted to use the two M.2 drivers mirrored in ZFS so.

Would the easiest thing to do would be install it all normally on the m.2 drives and just put simple grub install on a USB stick and stick it on the MB? I'm not over familiar with how FreeBSD handles ZFS on root with a different /boot partition so. I quickly tried doing it via a shell but its sufficiently different that I'm not sure what exactly is the best approach is.
The term M.2 drives is very ambiguous. There are M.2 mSATA/AHCI drives and there are M.2 NVMe drives.
I am struggling to understand why a modern bios would not boot from PCIe. That is what OptionROM does.
Are these paddle cards like m.2 NVMe to PCIe slot cards? What motherboard are you using?
It's an Intel Xeon x5600 series motherboard. It's a little old... I'm messing around with how the installer sets up the nvme drives after install and trying to replicate that on thr hard drives now.
Well, copying the partition tables from the m.2 drives, the first two partitions (efi and the 128k size one), then using gpart to install the bootloader on the harddrives got me to "gptzfsboot: no ZFS pools located, can't boot" so there's progress.