Following up, it seems that the problem is that I have hbamode enabled and the UEFI will not allow specifying one of the hbamode targets (/dev/sdX), but it will allow the internal sdcard. So I have a hybrid installation with the bootloader on internal sdcard and the installation on ZFS RAID1. Not perfect, but close enough.
root@pve104:~# blkid
/dev/mapper/pve-root: UUID="88a66d53-6bbf-495e-ae42-418ff6b6bad4" BLOCK_SIZE="4096" TYPE="ext4"
/dev/sdm3: UUID="IPnVEI-e3fG-cHhO-020i-d2rZ-WSES-PKDJed" TYPE="LVM2_member" PARTUUID="582f81c1-d85c-41b6-b910-fe9871df295e"
/dev/sdm2: UUID="6419-17A9" BLOCK_SIZE="512" TYPE="vfat" PARTUUID="c4a58082-0c33-473b-839f-aabeeb140b14"
/dev/sdb2: UUID="481B-47A2" BLOCK_SIZE="512" TYPE="vfat" PARTUUID="fbad3ff5-7acd-497e-9f26-8da399362d58"
/dev/sdb3: LABEL="rpool" UUID="8097517146150521578" UUID_SUB="6668182099607995460" BLOCK_SIZE="4096" TYPE="zfs_member" PARTUUID="84a92a92-2e7d-4fbb-b827-9744f5fcca3e"
/dev/sdk: UUID="E82B-543E" BLOCK_SIZE="512" TYPE="vfat"
/dev/sdi: UUID="E82B-543E" BLOCK_SIZE="512" TYPE="vfat"
/dev/mapper/pve-swap: UUID="eeb78737-3040-4ee9-82aa-64ea469ad71c" TYPE="swap"
/dev/sda2: UUID="7EE3-52DC" BLOCK_SIZE="512" TYPE="vfat" PARTUUID="cb39c2d0-cf79-44dd-9951-d6bda01b5964"
/dev/sda3: UUID="JN5xwq-TEMa-h8ff-yzw2-19Yo-fjP9-DUlhE2" TYPE="LVM2_member" PARTUUID="5c3e2f8b-3520-4643-a787-c1fe8b6e1565"
/dev/sdm1: PARTUUID="08ed9d4e-7357-440d-b1fd-00a52990aef1"
/dev/sdb1: PARTUUID="a86c47ed-7204-4991-b485-733eb95a6b18"
/dev/sda1: PARTUUID="71ba40da-2e2a-400e-822c-3b9d0cd4b039"
root@pve104:~# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINTS
sda 8:0 0 1.1T 0 disk
├─sda1 8:1 0 1007K 0 part
├─sda2 8:2 0 1G 0 part /boot/efi
└─sda3 8:3 0 1.1T 0 part
├─pve-swap 252:0 0 8G 0 lvm [SWAP]
├─pve-root 252:1 0 96G 0 lvm /
├─pve-data_tmeta 252:2 0 10G 0 lvm
│ └─pve-data 252:4 0 976.9G 0 lvm
└─pve-data_tdata 252:3 0 976.9G 0 lvm
└─pve-data 252:4 0 976.9G 0 lvm
sdb 8:16 0 1.1T 0 disk
├─sdb1 8:17 0 1007K 0 part
├─sdb2 8:18 0 1G 0 part
└─sdb3 8:19 0 1.1T 0 part
sdc 8:32 0 3.6T 0 disk
sdd 8:48 0 3.6T 0 disk
sde 8:64 0 3.6T 0 disk
sdf 8:80 0 3.6T 0 disk
sdg 8:96 0 3.6T 0 disk
sdh 8:112 0 3.6T 0 disk
sdi 8:128 0 3.6T 0 disk
sdj 8:144 0 3.6T 0 disk
sdk 8:160 0 3.6T 0 disk
sdl 8:176 0 3.6T 0 disk
sdm 8:192 0 29.2G 0 disk
├─sdm1 8:193 0 1007K 0 part
├─sdm2 8:194 0 512M 0 part
└─sdm3 8:195 0 28.7G 0 part
root@pve104:~#
-JB
