This thread mostly addresses the issue: Python API/highrescreenshot - #4 by mamoniem
I have found that the solution (using register_slate_pre_tick_callback) only works when being run from the Editor originally (and not using python’s subprocess package). Once I implemented this solution, I noticed that you still have to code a wait period into the script to let all the screenshots get processed. In my quest to have a dynamic wait period that is only as long as necessary, I employed register_slate_post_tick_callback in a similar way to register a loop for waiting, but so far it seems I don’t even need to encode any waiting, and seemingly the mere existence of the post tick callback makes the script wait for the previous pretick to finish.