Adding double stimulation doesn't make the program a lot more demanding and can be done long term. HOWEVER if the rest of your program is demanding that is another story. In other words if you are forced to change your program because of fatigue, it will not be because of the double stimulation part but because of your main workouts.
Personally I don't always do 3-4 weeks of hypertrophy. But yeah, after a 9-12 weeks cycle of hard training I normally go easier for 3 weeks. Either by doing pump work or technical work.
I try to always keep one big lift per session even during the lighter periods. I don't really use high reps for the big lifts, maybe up to 5-6 but I don't go all out. For example I might do sets of 3 reps with a weight I could do 5 reps with.
The assistance work is for higher reps (when doing an hypertrophy phase) and I'm even using some pump work using isolation exercises.