I'm installing Proxmox on an HP ProLiant DL360p Gen8 server, which cannot boot from NVMe. The NVMe drives are each installed on single-slot M.2 adapters in the rear PCIe slots.

1. Install Proxmox as "zfs raid1" (their installer uses wrong terminology) on two NVMe drives. Consider turning compression off. This Proxmox installation will be the actual instance that gets used, but is not currently bootable on this system.
2. Re-run the installer and install Proxmox on a flash drive as a single-drive zfs pool. I used 16GB, not sure if 8GB will suffice.
3. Reboot from the flash drive. This will load grub, but throw an error there are now two pools called rpool and one of them was last imported on another system. You'll be dropped into a BusyBox shell.
4. Import the NVMe-based pool by id. Run zpool import to list available pools, find the pool with two NVMe drives, and run zpool import -f 123456. exit the BusyBox shell, and the system will now boot the Proxmox instance installed in step 1.
5. Grab a Proxmox console, either by logging in locally (not ideal) or via the web UI. Run lsblk and you should see something like this:

# lsblk
NAME        MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda           8:0    1  14.9G  0 disk 
├─sda1        8:1    1  1007K  0 part 
├─sda2        8:2    1   512M  0 part 
└─sda3        8:3    1  13.5G  0 part 
sr0          11:0    1  1024M  0 rom  
nvme0n1     259:0    0 232.9G  0 disk 
├─nvme0n1p1 259:2    0  1007K  0 part 
├─nvme0n1p2 259:3    0   512M  0 part 
└─nvme0n1p3 259:4    0 232.4G  0 part 
nvme1n1     259:1    0 232.9G  0 disk 
├─nvme1n1p1 259:5    0  1007K  0 part 
├─nvme1n1p2 259:6    0   512M  0 part 
└─nvme1n1p3 259:7    0 232.4G  0 part

6. Import-and-rename the zpool on the USB stick, then destroy it. zpool import rpool deadpool; zpool destroy deadpool. (Since the NVMe-based rpool is already imported, this import of rpool is unambiguous, though importing by ID is probably smart, such as zpool import 654321 deadpool.)
7. Reboot and pray.
8. Cool it worked! Check zpool status. It probably alerts "Mismatch between pool hostid and system hostid on imported pool", since we're using the second installation's bootstraps with the first installation's zpool. Do the workaround: zpool set multihost=on rpool; zpool set multihost=off rpool. If you find yourself back at a BusyBox shell unable to automatically import the pool on the next reboot, force import it, then export and reimport it.

(initramfs) # zpool import -f rpool
(initramfs) # zpool export rpool
(initramfs) # zpool import rpool
(initramfs) # exit

9. Confirm zpool status is clean. Run a zpool scrub rpool if you want.

# zpool status
  pool: rpool
 state: ONLINE
  scan: scrub repaired 0B in 00:01:52 with 0 errors on Wed Feb  2 13:30:50 2022
config:

        NAME                                 STATE     READ WRITE CKSUM
        rpool                                ONLINE       0     0     0
          mirror-0                           ONLINE       0     0     0
            nvme-eui.0025385791b291a9-part3  ONLINE       0     0     0
            nvme-eui.0025385a9151127d-part3  ONLINE       0     0     0

errors: No known data errors

10. Use your NVMe-based Proxmox! I'm planning to migrate from a USB-based boot disk to an internal SD card (dd if=/dev/disk/by-id/usbstick of=/dev/disk/by-id/sdcard bs=1M) and then lock the SD card so the bootloader can't be overwritten.