I'm aware that you can do something like this to change the default place that the iPhone simulator saves screen shots:
defaults write com.apple.iphonesimulator ScreenShotSaveLocation "$HOME/Documents/Screenshots"
...but that seems to only change the default location for screen shots, not screen recordings (when you hold down "option" and record the screen as an mp4). These are still saved to the Desktop. I've tried the same command above but with ScreenRecordingSaveLocation, but that doesn't seem to work.
At this point I'm considering Automator/Shortcuts but I'm a noob to those and not sure how to do it.