Uh, the OP is using a helios64. My instructions only apply to that board (well, any rk3399 board).
No. The version of zfs in the debian repos does not support the 5.10 kernel. Install the proxmox kernel/remove debian kernel using omv-extras and you will be good to go.
Great! your right. I was to quick with my judgement thinking the problem is the same. It was only some time after I posted, I noticed the very first sentence of the OP. THen it hit me. My bad, sorry for that.
On the flipside. Your last reply solved the issue for me completely.
In OMV-Extras > Kernel > Install Proxmox kernel.
In OMV extras > Kernel > set as default (the pve kernel)
remove non-Proxmox kernels.
Check the current set kernel.
All 100% working.
Thank you for the latest post. It helped me a lot. Despite me starting off on the wrong foot.