Just wanted to update the thread with my experience. I've gone from "ready to throw in the towel" to "can't live without it"
I had what appeared to be a bad SD card. I got a new one and flashed it, and my reefbot has been stable ever since. It took some back and forth with support and some recalibrating, but I've got it to a point where it is less work to have it perform my and my test results are reliable and consistent.
A few things that improved my experience: recalibrate once every few weeks, put two stirrer beans in the test chamber, replaced the OEM SD card with a SanDisk SD card, and work with support on any results that are not consistent. I also have found API Ca, RedSea Kh, RedSea PO4, and Tropic Marin NO3 to be most consistent and the best use of space.