Safe to enable dnodesize=auto on rpool/data?

How is your system booted - does it use proxmox-boot-tool (check with `proxmox-boot-tool status`) - if yes you should be safe since the kernel and initrd are read from the ESP(vfat) partition.
If your system has been installed a long time ago and does not use proxmox-boot-tool then enabling a read-write feature on the pool might render it unbootable!

see also https://pve.proxmox.com/wiki/ZFS:_Switch_Legacy-Boot_to_Proxmox_Boot_Tool
 
Code:
root@vmhost:~# proxmox-boot-tool status
Re-executing '/usr/sbin/proxmox-boot-tool' in new private mount namespace..
System currently booted with uefi
C606-33DC is configured with: uefi (versions: 5.13.19-6-pve, 5.15.107-2-pve, 5.15.108-1-pve)
C606-9E7D is configured with: uefi (versions: 5.13.19-6-pve, 5.15.107-2-pve, 5.15.108-1-pve)
C606-DEEA is configured with: uefi (versions: 5.13.19-6-pve, 5.15.107-2-pve, 5.15.108-1-pve)
C607-1EE4 is configured with: uefi (versions: 5.13.19-6-pve, 5.15.107-2-pve, 5.15.108-1-pve)
C607-5E11 is configured with: uefi (versions: 5.13.19-6-pve, 5.15.107-2-pve, 5.15.108-1-pve)
C607-9CE4 is configured with: uefi (versions: 5.13.19-6-pve, 5.15.107-2-pve, 5.15.108-1-pve)
C607-DEA3 is configured with: uefi (versions: 5.13.19-6-pve, 5.15.107-2-pve, 5.15.108-1-pve)
C608-1C88 is configured with: uefi (versions: 5.13.19-6-pve, 5.15.107-2-pve, 5.15.108-1-pve)