The great news is you’ve got the speed, plenty struggle with that! I’m not sure exactly if your problem is tracking or the string changes in general. One thing you could try is a forward chaining pattern where you gradually introduce more notes into it. Sort of like a “near tremolo” where you gradually add a note at a time until the tremolo is gone (but this helps to maintain the tremolo “motion” throughout, which is key)
Start with
U D U D U D U D U D U D U D U D U D U D U D U D
e||:15-12-12-12-12-12|15-12-12-12-12-12|15-12-12-12-12-12|15-12-12-12-12-12:||
Make sure to use a pretty powerful accent on the first note of each 6 note group. Troy mentions this in the “chunking” video. I’ve heard others recommend it as well. @Tom_Gilroy preaches it extensively and even dares his students to “break a string” on their accents. I’ve heard Claus Levin say “if you’re practicing without accents, you’re wasting your time.” It’s critical for hand sync, a strong sense of rhythm and chunking in general. It helps you brain focus on doing one thing (the accent) while the “spasm” of notes that follows it happens on it’s own.
Anyway, then, start adding notes 1 at a time until you get the full phrase
U D U D U D U D U D U D U D U D U D U D U D U D
e||:15-12------------|15-12------------|15-12------------|15-12------------:||
B||:------15-15-15-15|------15-15-15-15|------15-15-15-15|------15-15-15-15:||
Next
U D U D U D U D U D U D U D U D U D U D U D U D
e||:15-12------------|15-12------------|15-12------------|15-12------------:||
B||:------15-12-12-12|------15-12-12-12|------15-12-12-12|------15-12-12-12:||
Next
U D U D U D U D U D U D U D U D U D U D U D U D
e||:15-12------------|15-12------------|15-12------------|15-12------------:||
B||:------15-12------|------15-12------|------15-12------|------15-12------:||
G||:------------14-14|------------14-14|------------14-14|------------14-14:||
Finally
U D U D U D U D U D U D U D U D U D U D U D U D
e||:15-12------------|15-12------------|15-12------------|15-12------------:||
B||:------15-12------|------15-12------|------15-12------|------15-12------:||
G||:------------14-12|------------14-12|------------14-12|------------14-12:||
You can even try playing this along with Troy at various speeds (personally I wouldn’t spend much time below 85%, other than to just initially “lock in” and make sure you can feel the pulse. I like using the “waveform” option in sloundslice because it gives greater flexibility to set the loop points exactly where they need to be to line up with the beats. When it’s done in the “score” view it’s not going to line up directly with the measures just due to how well the “sync” points were set.
I tried that out just now, and I don’t even practice starting on an upstroke very often. I was able to get to the 90% tempo in just a few minutes so I’m sure with diligent work you could get this 100% without too much effort.