Well I can't say I have a magic solution, but I can say that my experience is similar to yours, and I will start by underscoring some other answers already given which I too have found both accurate and helpful. First, I too have experienced that this floating fist works better with the wrist doing most of the work at slower tempos, with more arm / elbow motion becoming more prevalent at higher tempos. There is almost a kind of gear shift involved that I've just come to accept. Interestingly for me, it is the middle tempos where I'm trying to get a strong swing feel that give me the most trouble, almost as if it is an awkward place where neither "gear" is perfect for the tempo, but I digress.
One compromise that seems to have helped me with the anchoring is one I'm not super happy with, because it makes me more dependent on the guitar setup, but it does work. My "anchoring" consists of keeping my fist mostly closed, but allowing some part of my closed pinky to rest on the pick guard. To make this ideal for me, I've had to first choose guitars that have pick guards (Les Paul is my favorite), AND... adjust the height of the pickguard, sometimes employing extra hardware and washers, so it is just a tad lower then the height of the first string. Just low enough that when you pick the high string, you don't hit the pick guard with your pick.
So the result is that at lower tempos, I can keep my hand completely floating if I want. But as I get to those upper middle tempos where I tend to get excessively bouncy, I can begin to rest by putting slight pressure on the last knuckle of my pinky against that pickguard. This helps in two ways... during practice, it helps me to better refine my movements, so the "bounce" needed to alternate pick without hitting strings you want to cross over, is minimal. And because I haven't really changed hand position, if I want to practice getting the cross string clearance with a little twisting motion of the wrist, I can simply lift the whole hand up, bend my wrist down slightly, and work that way. Then as I slowly increase the metronome speed during practice, I can use that slight pinky-knuckle anchor a little more, to help me feel out the best balance of moving the pick with my fingers, wrist, elbow, and even upper arm. The second way this anchoring helps is during performance. Even if I warm up before I go "on", I've found the two most important things I can to to be at my best is (1) get the pick properly situated in my hand, and (2) find that ideal picking height and motion that gets me reasonably fluid, without having to get half way through the first set to be that way. And for me, that slight resting on the pickguard, as I've described, seems to give me the best chance of quickly getting into a comfort zone. As others have pointed out, its all about feedback and reference.
Aside from the fact that this kind of anchoring requires a properly adjusted pick guard, the other initial disadvantage is that when you get down to the lowest strings, the tip of your pinky is now coming into contact with the high string. But trust me, that's just a temporary problem. In time you just naturally learn to let the pinky relax "out" a little more as you get to the lower strings, and it becomes more or less automatic.
I know there will be people that will chime in and dictate all the more "proper" ways to pick... some will say ONLY use the wrist... others NEVER twist, and alternately ALWAYS anchor, or NEVER anchor. Its always good to try what other guitarists have found works for them, to avoid missing an opportunity to learn. In fact, since your technique (and issues) sound similar to mine, I'd like to hear of other solutions you find. But bottom line, its most important to continually find creative new ways around your own problems. Lets face it... Jeff Healey didn't learn to play good slide guitar with his feet, by listening to players who's personal success turned them into dictators of "proper" technique.