@Markus Braun
edit: I have an orange pi 5 plus!
I have mine booting without any SD card. I sadly don't recall whatever guide I followed but I am pretty sure I had to toogle nvme booting somwhere in armbian-config or some command that updated the bootloader on the opi5+
This is my /boot/armbianEnv.txt
verbosity=1
bootlogo=true
overlay_prefix=rockchip-rk3588
fdtfile=rockchip/rk3588-orangepi-5-plus.dtb
rootdev=UUID=4578f871-5c8a-406a-87bd-be8562b1f8bf
rootfstype=ext4
usbstoragequirks=0x2537:0x1066:u,0x2537:0x1068:u
my /etc/fstab look like this
UID=4578f871-5c8a-406a-87bd-be8562b1f8bf / ext4 defaults,noatime,commit=600,errors=remount-ro 0 1
UUID=dce8fefd-ace9-468e-8695-55071f59094f /boot ext4 defaults,commit=600,errors=remount-ro 0 2
tmpfs /tmp tmpfs defaults,nosuid 0 0
[... nfs and other irrelevant junk]
Did you change your fstab before running dd so it reflects the UUID of the the nvmes /boot disk? or change it to /dev/nvme... blabla ?
I will be getting a new opi5+ in a week or two with an nvme so I will need to revisit these steps anyway. Let me know if I can check something on my side.