Hi. Just got here, and don't have any interest whatsover in catching up on 8 pages of jibber-jabber. If you're still looking for the answer to the Q posed in the OP, it's this:
If I, as a coach/training partner, hit you a rally ball that is perfectly in your wheelhouse -- the right height, the right position relative to your body, exactly right for you to have time to set up properly and take a perfect swing -- can you hit that ball authoritatively where and how you want it, pretty much every time?
If "yes," then going up levels is a matter not of improving your stroke, but rather your ability to get yourself into perfect position to hit exactly THAT stroke, every shot, for the rest of your life. And that's 100% fitness and footwork.
You want to jump levels in a hurry? Here's a foolproof prescription: One hour doing a light jog on a treadmill, four or five days a week, for three months, just to build up your gas tank (I suggest winter for this). Then, half an hour a day, doing wind sprints, three days a week, for the other nine months out of the year (with an occasional hour jog once every two weeks or so, just to keep the gas tank full). Rinse, repeat, year after year. Without doing any real work beyond maintenance on your strokes, they'll become things of legend, since you'll be in position to hit each of them perfectly, every time. Do a Google image search on the phrase "tennis player abs." You'll know you're getting there when you look like that when you lift your shirt up. For tennis players, that's not a vanity look -- that's pure form following function. Pros get to every ball that isn't a 7.0 level winner, because they can.
That, and your serve. There's no shortcut on the serve. Though I would change my strategy. How tall are you? I think somebody did you a disservice by suggesting topspin/kick to you. Kick serves are a tall man's game. We think of them as kicking "up," but they actually don't. They kick forward, and/or to the side. There's nothing on a flat, horizontal court surface for them to kick up off of. They give the illusion of kicking up because they lose less of the bounce's vertical momentum to surface drag on the bounce than flat or slice serves, so they bounce higher than those do for everybody. But they still only bounce to a natural height determined by the position from which they're struck. That's why we ooh and aah at an Isner kick serve. Not because he's hitting it with way more kick-spin than David Ferrer, but because he's hitting it twelve feet in the air, so it's bouncing over heads on the other side. When a shorter player hits these, they just sit there in the primo striking zone. They're really only a valid option below about 6'0" if they're the only kind of second serve you're capable of getting in the box. Try integrating more slice and less top, and even if your serve doesn't "look" as whiz-bang cool, I think you'll see a marked improvement in your results with it.
