Lack of TRIM shouldn't be a huge issue in the medium term. It's pretty likely that you'll be able to flip the trim support bit on that pool within the next year and a half (ZoL 0.8.0 is in the pre-release stage now and includes TRIM,) and I don't see you writing enough data to it in that time to trash the drive. If you wanted to reduce wear you could size the filesystem so that there's about 20% free space at the end of the drive, then the firmware will use that for garbage collection. It looks like you're already doing that by setting hdsize=80, IMO that's a fine mitigation until TRIM is supported.
You might, and I say might because I don't know if this is accurate, want to use ashift=13 on your Samsung drive. I think their 3d vnand uses 8kb pages, but I need someone more knowledgeable to confirm. I have a stack of 850 Pros over here and I've been researching this the last week or so.