Based on suggestions from others in this thread, I've looked into this further and to get "centimeter-level" precision (which is what I think we're looking for) you'll need an RTK drone which will use a ground station to improve its location positioning. From what I understand you also need to subscribe to an RTK service, which in my area is over $1,000/year (apparently it's free in some states that provide it as a public service). A DJI dealer told me I didn't need the RTK service, but after pushing on this he checked and later told me it's required. DJI's new Mavic 2 Enterprise Advanced has an RTK option, but the drone starts at about $6.5K and so it's a major upgrade to get this level of precision. Purely based on its name, it would appear the Phantom 4 RTK has this capability, but it also looks to start in the $6K range.
For what it's worth, I recently received a replacement M2Z as part of the "Refresh" program and I had about the same level of altitude variability within a day of taking it out of the box, suggesting to me it's not a unit-specific issue.
