A well known book on sports coaching called "The inner game of tennis" covers this issue rather well. Basically it advocates concentrating on one or two key issues (I find steady approach speed is a good one personally, followed by angle to the touchdown point) and much of the other stuff magically seems to fall into place.
Never been a QFI, but I used to do a lot of sports coaching (not tennis) where I applied the principle and seemed by and large to get the results I wanted. Trick was to know which core skills to concentrate on, and it wasn't always the same for each person.
G