I start this software #top -f -F 1
And then with "L" I change to see all disks and CPU. In the benchmark time you can see disk activity and you can compare do all disk are busy equally and what is CPU activity.
ZFS is very complex system. It is COW system. If you don't care compression, encryption, snapshot, data integrity then use old file systems.
Use #atop to see CPU and disk usage. Maybe it will show something interesting.
If you want to speed up write you can set up sync=disabled But keep in mind you can lose some data in the power outage
as of VM look here - https://kb.blockbridge.com/technote/proxmox-optimizing-windows-server/part-2.html
You can download deb directly - http://ftp.no.debian.org/debian/pool/main/d/debsums/debsums_184.108.40.206_all.deb
Using Live distro you can mount rpool and then bind mount /dev /sys /proc , chroot and use it as normal OS. In that environment you can install debsums, look at files and do changes.