Be aware if you have the iGPU as the only video output device and passthrough to a VM it will no longer show what the host system is doing. This would be referred to as a headless server. I would suggest making sure you can SSH into the host before doing that. LearnLinuxTV has guides for how to do that with best security practices.

Docker works differently, so it may not be an issue with that.

Truenas uses KVM for virtual machines. So that will allow GPU passthrough, but may require command line and config files to do it. For docker this seems relevant: https://forums.truenas.com/t/electric-eel-nvidia-gpu-passthrough-support/11797

When you read files from the ZFS filesystem it will automatically keep the files in RAM. This is called the ARC and it is why people frequently recommend having a lot of RAM with ZFS. The ARC is very effective, automatic, and has no risk because it only caches reads. A cache drive is a secondary ARC generally using a fast SSD. The problem is that it generally only helps performance when you are reading lots of small files multiple times. This is because ZFS does so well reading large files from HDD that it doesn’t make much of a difference.

In short: If you already have the drive and want to play with the feature, go for it. But if your going to spend money on the drive, you will probably be better served spending it on more RAM.

I assume the cache drive is for your ZFS pool. You probably don’t need that.