I looked at doing two vdevs but was put off by the lower usable storage. At a certain point, maybe that's not as important as I think though.
Yeah, the choice for 6TB wasn't my best. I got the two older drives a few years back on a Newegg flash sale, and it seemed like plenty, especially considering Unraid's model of 1 parity drive and 100% usable storage on the data drive(s). Then, when I decided to upgrade, I was too cheap to go buy 4 whole new drives, so I just went with more of what I already had (to add insult to injury, they're all WD Red drives...).
Part of me is starting to wonder, honestly. I will say that the web UI for TrueNAS Scale is leagues better than Unraid's, which to me always felt confusing and hacked together. ZFS is also really nice, although Unraid did recently add support.
One pain point I've run into with TNS is that access to Docker or Kubernetes seems to be intentionally locked down from access anywhere but the built in apps catalog. As someone who works with Docker and various orchestration engines professionally, I much prefer being able to define and stand up my own services to using a list of predefined templates. There are obviously ways of getting around the restrictions in TNS, but with Unraid, I could install something like Portainer or simply drop into the terminal and run docker commands myself. Not having that is frustrating.
Overall though, I think TrueNAS is a much cleaner and more modern user experience, so long as you stay on their rails. Which I suppose is the point.