Hmm for me that mechanic never worked properly on m/s and is a waste of time 😆
For me it worked always pretty well. You just need to cast shot in your ingame macro, instead of in your lua and cast the debuff first in your lua. Then it's applied before the actual shot hits the target. Also since fire rose doesn't trigger gcd, you could rely on that for the entire trash fights. At least that's how I did it until the latest changes regarding shot cd and thunderstorm. Haven't played it since then.