I’m a lifelong elbow picker, though I’ve worked really hard over the last year to become a wrist picker. Things that I worked on early on and taught my students was to work independently with each hand.
You know that your right hand is fast, you’ve proved it. Your synchronization will come, but get your left hand up to speed. Do this by just drilling simple 3nps patterns with hammer on and pull offs. Set the metronome at 110bpm, and play sextuplets legato. Just play the major scale (or any of the modes of it) and repeat twice on each string the 3np pattern. Down pick the first time, up stroke the second, move to the next string. Really hit the down stroke hard.
The next step is stay on one string and alternate, legato twice, picking twice.
Lastly, play 16th note picked patterns at 180. On the G string (or any), play 11 12 14 11 - 12 rest. The pattern starts and ends on a down stroke on the beat. You can play this fast with your right hand, you’ve proved it. If you can’t get it to work, try it legato, picking the 11 downstroke and the last 12 downstroke. Work on keeping it in time if you play legato.
Once you have these concepts, you can move on to changing strings, using the knowledge that changing after playing a downstroke will be easier.
I want to add, the reason I’m working on wrist picking is that I’ve always struggled with what I call the in-between speeds. Between 115-150bpm 16ths, picking across strings in time with elbow motion was really hard. The tempo just felt like the locked in feeling of elbow was too tight, to move that slow. I’ve watched 100s of hours of video and most elbow pickers play wrist style at medium tempos, then lock it to elbow at higher speeds. Michael Angelo is maybe the one exception. Since I’ve switched to wrist motion, I’ve been able to fill in that in-between with wrist and go back to elbow for high speed (see Vinnie Moore’s instructional, he does this too).
