It looks like you are using legacy bios. mine is using uefi with a zfs rpool

proxmox-boot-tool status
Re-executing '/usr/sbin/proxmox-boot-tool' in new private mount namespace..
System currently booted with uefi
31FA-87E2 is configured with: uefi (versions: 6.5.11-8-pve, 6.5.13-5-pve)

However, like with everything a method always exists to get it done. Or not if you are concerned.

If you are interested it would look like…

Pool Upgrade

sudo zpool upgrade <pool_name>

Confirm Upgrade

sudo zpool status

Refresh boot config

sudo pveboot-tool refresh

Confirm Boot configuration

cat /boot/grub/grub.cfg

You are looking for directives like this to see if they are indeed pointing at your existing rpool

root=ZFS=rpool/ROOT/pve-1 boot=zfs quiet

here is my file if it helps you compare…

#
# DO NOT EDIT THIS FILE
#
# It is automatically generated by grub-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
#

### BEGIN /etc/grub.d/000_proxmox_boot_header ###
#
# This system is booted via proxmox-boot-tool! The grub-config used when
# booting from the disks configured with proxmox-boot-tool resides on the vfat
# partitions with UUIDs listed in /etc/kernel/proxmox-boot-uuids.
# /boot/grub/grub.cfg is NOT read when booting from those disk!
### END /etc/grub.d/000_proxmox_boot_header ###

### BEGIN /etc/grub.d/00_header ###
if [ -s $prefix/grubenv ]; then
  set have_grubenv=true
  load_env
fi
if [ "${next_entry}" ] ; then
   set default="${next_entry}"
   set next_entry=
   save_env next_entry
   set boot_once=true
else
   set default="0"
fi

if [ x"${feature_menuentry_id}" = xy ]; then
  menuentry_id_option="--id"
else
  menuentry_id_option=""
fi

export menuentry_id_option

if [ "${prev_saved_entry}" ]; then
  set saved_entry="${prev_saved_entry}"
  save_env saved_entry
  set prev_saved_entry=
  save_env prev_saved_entry
  set boot_once=true
fi

function savedefault {
  if [ -z "${boot_once}" ]; then
    saved_entry="${chosen}"
    save_env saved_entry
  fi
}
function load_video {
  if [ x$feature_all_video_module = xy ]; then
    insmod all_video
  else
    insmod efi_gop
    insmod efi_uga
    insmod ieee1275_fb
    insmod vbe
    insmod vga
    insmod video_bochs
    insmod video_cirrus
  fi
}

if loadfont unicode ; then
  set gfxmode=auto
  load_video
  insmod gfxterm
  set locale_dir=$prefix/locale
  set lang=en_US
  insmod gettext
fi
terminal_output gfxterm
if [ "${recordfail}" = 1 ] ; then
  set timeout=30
else
  if [ x$feature_timeout_style = xy ] ; then
    set timeout_style=menu
    set timeout=5
  # Fallback normal timeout code in case the timeout_style feature is
  # unavailable.
  else
    set timeout=5
  fi
fi
### END /etc/grub.d/00_header ###

### BEGIN /etc/grub.d/05_debian_theme ###
set menu_color_normal=cyan/blue
set menu_color_highlight=white/blue
### END /etc/grub.d/05_debian_theme ###

### BEGIN /etc/grub.d/10_linux ###
function gfxmode {
        set gfxpayload="${1}"
}
set linux_gfx_mode=
export linux_gfx_mode
menuentry 'Proxmox VE GNU/Linux' --class proxmox --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-/dev/sdc3' {
        load_video
        insmod gzio
        if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
        insmod part_gpt
        echo    'Loading Linux 6.5.13-5-pve ...'
        linux   /ROOT/pve-1@/boot/vmlinuz-6.5.13-5-pve root=ZFS=/ROOT/pve-1 ro       root=ZFS=rpool/ROOT/pve-1 boot=zfs quiet
        echo    'Loading initial ramdisk ...'
        initrd  /ROOT/pve-1@/boot/initrd.img-6.5.13-5-pve
}
submenu 'Advanced options for Proxmox VE GNU/Linux' $menuentry_id_option 'gnulinux-advanced-/dev/sdc3' {
        menuentry 'Proxmox VE GNU/Linux, with Linux 6.5.13-5-pve' --class proxmox --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-6.5.13-5-pve-advanced-/dev/sdc3' {
                load_video
                insmod gzio
                if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
                insmod part_gpt
                echo    'Loading Linux 6.5.13-5-pve ...'
                linux   /ROOT/pve-1@/boot/vmlinuz-6.5.13-5-pve root=ZFS=/ROOT/pve-1 ro       root=ZFS=rpool/ROOT/pve-1 boot=zfs quiet
                echo    'Loading initial ramdisk ...'
                initrd  /ROOT/pve-1@/boot/initrd.img-6.5.13-5-pve
        }
        menuentry 'Proxmox VE GNU/Linux, with Linux 6.5.13-5-pve (recovery mode)' --class proxmox --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-6.5.13-5-pve-recovery-/dev/sdc3' {
                load_video
                insmod gzio
                if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
                insmod part_gpt
                echo    'Loading Linux 6.5.13-5-pve ...'
                linux   /ROOT/pve-1@/boot/vmlinuz-6.5.13-5-pve root=ZFS=/ROOT/pve-1 ro single       root=ZFS=rpool/ROOT/pve-1 boot=zfs
                echo    'Loading initial ramdisk ...'
                initrd  /ROOT/pve-1@/boot/initrd.img-6.5.13-5-pve
        }
        menuentry 'Proxmox VE GNU/Linux, with Linux 6.5.11-8-pve' --class proxmox --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-6.5.11-8-pve-advanced-/dev/sdc3' {
                load_video
                insmod gzio
                if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
                insmod part_gpt
                echo    'Loading Linux 6.5.11-8-pve ...'
                linux   /ROOT/pve-1@/boot/vmlinuz-6.5.11-8-pve root=ZFS=/ROOT/pve-1 ro       root=ZFS=rpool/ROOT/pve-1 boot=zfs quiet
                echo    'Loading initial ramdisk ...'
                initrd  /ROOT/pve-1@/boot/initrd.img-6.5.11-8-pve
        }
        menuentry 'Proxmox VE GNU/Linux, with Linux 6.5.11-8-pve (recovery mode)' --class proxmox --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-6.5.11-8-pve-recovery-/dev/sdc3' {
                load_video
                insmod gzio
                if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
                insmod part_gpt
                echo    'Loading Linux 6.5.11-8-pve ...'
                linux   /ROOT/pve-1@/boot/vmlinuz-6.5.11-8-pve root=ZFS=/ROOT/pve-1 ro single       root=ZFS=rpool/ROOT/pve-1 boot=zfs
                echo    'Loading initial ramdisk ...'
                initrd  /ROOT/pve-1@/boot/initrd.img-6.5.11-8-pve
        }
}

### END /etc/grub.d/10_linux ###

### BEGIN /etc/grub.d/20_linux_xen ###

### END /etc/grub.d/20_linux_xen ###

### BEGIN /etc/grub.d/20_memtest86+ ###
### END /etc/grub.d/20_memtest86+ ###

### BEGIN /etc/grub.d/30_os-prober ###
### END /etc/grub.d/30_os-prober ###

### BEGIN /etc/grub.d/30_uefi-firmware ###
menuentry 'UEFI Firmware Settings' $menuentry_id_option 'uefi-firmware' {
        fwsetup
}
### END /etc/grub.d/30_uefi-firmware ###

### BEGIN /etc/grub.d/40_custom ###
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.
### END /etc/grub.d/40_custom ###

### BEGIN /etc/grub.d/41_custom ###
if [ -f  ${config_directory}/custom.cfg ]; then
  source ${config_directory}/custom.cfg
elif [ -z "${config_directory}" -a -f  $prefix/custom.cfg ]; then
  source $prefix/custom.cfg
fi
### END /etc/grub.d/41_custom ###

You can see the lines by the linux sections.