Update: This afternoon, using the same DP cables, I connected each monitor directly to one of the two graphics cards, and the setup is way more stable, both when initially connecting all the monitors, and when using Monitor Profile Switcher to switch between configurations. So it looks like the issue was using DP MST daisy chaining. The actual connection to the daisy chained monitors was fine, but overall it created a stability issue. I'll probably add a 3rd 4k monitor in the next few weeks and remove 1 or 2 of the 2k monitors, and I'm guessing the existing cabling will be fine, though I may need a longer cable for the monitor farthest from the PC to get full standing height desk functionality. Thanks for the help guys.