Just not sure if :ACQuire:MDEPth? is the right waveform point when reading the screen waveform.
In the set of commands you posted, MDEPTH doesn't matter, it is ignored.
If you set normal mode with
:WAV:MODE NORM, that means at the next
:WAV:DATA? it will transfer only what is on the display at that moment. It doesn't matter the MDEPTH and the start/stop points you set. Those are for RAW mode transfer. In NORMal mode transfer, only 1200 samples will be downloaded, each sample corresponding to a pixel from the trace on the oscilloscope's display (display width is 1200).
If the trigger moment is in the middle of the screen, then you can discard all samples except the sample 600
th sample, and that one would be the 8 bits value at the trigger time.
If you want (rare) but precise (aperture), you can put the oscilloscope in single sweep mode (
:TRIGger:SWEep SINGle), then generate from the PC a pulse (or from some other device with a digital output), and that pulse will trigger the oscilloscope (you apply that pulse to another osc channel), then download only that screen captured after you sent the trigger pulse (
:WAV:MODE NORM; :WAV:DATA?). Then, in the PC software, discard all other points in the trace, and keep only the ADC sample from the trigger moment.
If the trigger moment is not critical (for reading once at 1..5 seconds or so), then you can use the measurement functions of the oscilloscope (the ones on the buttons at the left side of the screen). Though those functions are using all the pixels of a trace, but only the pixels that are visible on the osc screen (1200 visible values of the trace, not the full range of samples given by memdepth). For example
:MEASure:ITEM VAVG,CHANnel1 if the voltage is mostly DC in the screen around the trigger time.