By exprience with Trunas has been that ZFS does not like virtual disks. Especially when the Proxmox host also uses ZFS. Two layers of ZFS arc caching creates some memory issues. Setting the Host datasets to Metadata only may help.

But the most reliable method would be doing hardware passthrough of physical disks to the VM. It gets you most of the bare metal reliability benefits without having to commit the entire hardware box to one OS.

You may also want to disable memory ballooning in your VMs. It works well when you have lots of small VMs, but if you have a few large ones, it can cause issues if you overallocate Ram to VMs, beyond what the OS has available. I suspect it could also be interferring the zfs arc as well.

Lastly, check that your VM is set to use the “host” Cpu type. Freenas would likely benefit from having access to more CPU functions.