I'm using a RaidZ2 without any issues, but I'm also not using any of the apps, since I'm running an Ubuntu VM with docker for all my apps. I didn't follow all the discussions but it seems that destroying the first pool created results in apps become unusable until some paths/settings are fixed in TrueNas, but don't quote me on that, maybe someone else has more information about this.
To add to @Mobius explanations about the Vdevs:
Once you loose all the drives from 1 vdev you loose everything. Meaning with 3x2 drive vdevs you can only loose 1 drive per vdev (but 3 drives in total if they are all from different vdevs) and with 2x3 drive vdevs you can loose 2 drives per vdev without loosing any data.
@L-Pif you have 10 gbit/s networking, you might want to look into adding an nvme cache drive as an L2ARC. ARC is a read only cache, which is keeping the files which you might need next in a cache in memory for quick access. You can extend this with an L2ARC cache which, stores additional files on a dedicated drive. If this drive is an SSD it's gonna ready and serve those files significantly faster then if Hexos needed to read them from the slow HDDs. This might make sense in your case if you have 10gbit/s networking, everything below and the speed benefits are likely to be marginal at best.
And no there is no write cache in Hexos only a read cache. 🙂