This will be a short one since I have moved on from this configuration

A year or two ago I wanted to have redundant SSD's in pfSense, and I happened to find some very cheap 32GB Hynix M.s SATA SSD's. I bought an M.2 to SATA adapters and threw them in. I configured them with ZFS and put them in a GEOM Mirror within pfSense

Here you can see where I placed them in the system, and how I managed the cabling

After a few months, one of the SSD's dropped out of the mirror, so I threw in a spare SSD I had laying around

The other small Hynix SSD ended up failing too, and I moved away from this setup. While I was investigating the problem with the SSD's dropping out of the mirror, I noticed that there was a LOT of data written to them, and they had already gotten half way to their TBW within a few months. It turns out pfSense is quite hard on SSD's, and these SSD's simply weren't durable enough

Eventually I fixed the issue, and you can read about it on the latter part of my blog post here. Its a simple fix, and in my opinion should be the default configuration

Overall I did like this setup, and if you follow the above links advise an enable RAMDisk, I can see this being a viable setup. There was no downtime when swapping out the SSD's and rebuilding the mirror