Booting System76 Darter Pro/coreboot from external SDD...?

After my old laptop developed what seems to be NVME controller faults, I installed my backup onto an external SDD. On a borrowed Dell Latitude 5510 that external SDD boots. The SSD is partitioned as

Disk /dev/sdb: 1.82 TiB, 2000365371904 bytes, 3906963617 sectors
Disk model: My Passport 264F
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 1048576 bytes
Disklabel type: gpt
Disk identifier: AAEEB6DD-E581-4EC4-9D54-06665DCB89CF

Device          Start        End    Sectors  Size Type
/dev/sdb1        2048     616447     614400  300M EFI System
/dev/sdb2      616448 3839854591 3839238144  1.8T Linux filesystem
/dev/sdb3  3839854592 3906963455   67108864   32G Linux swap

The EFI System partition gets mounted as /boot/efi and there’s GRUB installed onto the SSD to boot.
When I plug the SSD into the Darter coreboot sees the external drive, but when I try to boot from it, it does not find anything to boot… (I do not remember the exact error message). From a USB memory stick that has the Manjaro installation system on it, the machine can also not boot. But it did boot from a USB stick with Ventoy on it, then booting into the Manjaro installation system.
So, my question is, what specific tweak is necessary to boot my external SSD that currently has my working system/installation on it?

I have no experience with CoreBoot, but one of the usual causes of a failure to boot is legacy BIOS emulation. When that is active, the machine will attempt to boot in legacy BIOS mode, and that won’t work on an EFI system.

I’ve searched some more and found this: Firmware Open - UEFI docs. It seems this only loads EFI/BOOT/BOOTX64.EFI. Then the question would be, can I just copy my manjaro directory under EFI, or is there some grub-install or similar command that needs to be used?

I would advise grub-install, because it then also sets the EFI variables in the UEFI CMOS memory.