You do not have to train heavy, as in near to your 1RM. Executing a rep with the intent to accelerate, i.e, maximal force without sacrificing contraction of the target muscle regardless of the weight used will maximally recruit most fibers. Think about sprinters. If the amount of resistance used is the sole factor for fast twitch fiber recruitment, they would be using mostly slow twitch fibers while sprinting.
The other way would be going to failure without maximally accelerating each rep. Regardless of the load used, most fibers will be activated once you reach failure as long as you know how to activate the tatget muscle in the first place(mmc). This is the body's survival mechanism.
Using maximal acceleration, you can avoid taking too many sets to failure since the objective is to recruit and fatigue all the muscle fibers. If you are too fatigued from your workouts, perhaps this may be the way to go.
Sarcoplasmic hypertrophy occurring disproportionately from skeletal muscle growth is just a theory. It has never been proven. What we experience from dropsets and high reps can simply be transient fluid retention in the muscle. Chasing a pump with these intensity methods does have value but I doubt it has anything to do with sarcoplasmic hyperyrophy.