I'm pretty new to the Mini 2, and have discovered that it only seems to show a very simple relative-elevation, which is just the delta elevation from the point the drone took off from.

I have a 250' tall ridge line 1/3 mile from my home, and if I want to fly, say, 300' above the ridge, I am then 550' above the take off elevation.  So I have to acknowledge the elevation warning.  I also don't have any way of knowing the actual altitude over ground of the drone at all.  It seems to me that if the DJI Fly app was online, and is showing map imagery coming off the internet, then it would be trivial for it to calculate an estimated altitude by checking the ground height at its current GPS location vs the GPS elevation of the drone.

Just want to make sure I'm not missing a setting or doing something wrong here, or if I'm totally on my own as to the actual altitude above ground when flying on land that isn't perfectly flat like the Great Salt Lake or something.

Yes, you are totally on your own....  ;-)

Height shown in the FlyApp is the barometer height, zero`d at take-off so during flight height indication (plus or minus) are relative to where you started your flight.
Using GPS ect for height...well  DJI did not incorporate any other eay of knowing the fly height.
Problem? nah, well not for me.

My wish only is that DJI would show also the bottom sensor measured height in the FlyApp!

cheers
JJB