Stuck between elbow and wrist motion

I'm stuck between two techniques. I can play fairly quick tremolo on 1 string without fatigue (up to 180 or so), using what I think is elbow motion, but it's very difficult to change strings or do anything requiring more complex articulation (works great for Thunderstruck, but not much else). At slower tempos (16th notes around 110, or triplets around 140), I can play comfortably with good articulation using some kind of wrist motion. I've been working on it with a metronome, and trying USX and DSX depending on the string changes. But I can't seem to get any faster at this - fatigue sets in quickly, and it feels like a lot of effort to generate any speed just using my wrist. To do the fast tremolo cleanly, I have to bend my thumb down to put the pick at a steep angle to the string. It works, but it feels like a lot of tension in my hand. It's also hard to play this way at slower tempos. Whatever motion I'm using in the bluegrass tunes is what I'd like to build my technique on, as it feels much more usable (for timing, string changes, tone, etc.) than the arm tremolo. I want to be able to scale this up to higher speeds, without having to change where the motion is coming from. How should I proceed? Thanks for any tips! Jeff