From what I can find, the feature is still very new. I expect others may have this issue.
Until an update fixes this for you, I suggest taking the time to do a little practice with your camera.
-Practice triggering the shutter release quickly for a snapshot.
-also practice holding down the shutter for a video.
-additionally, practice holding the shutter to do a QuickTake. and then slide your finger and the red circle turns to a square so you get to remove your finger and keep shooting video. That's cool but it's the opposite of what you are asking for in you question.
-and also additionally, practice doing a QuickTake and pressing volume and you will shoot a picture while shooting video! That is a bit closer to your issue.
and finally practice deleting all these files you make when you practice using your camera. It may be useful to sync it all to iCloud and then use a bigger machine to do all the deleting. That way you can simply shoot away to your hearts content and not worry about filling up your phone. Just take the time to clean it up later.