x3, we have similar stats (weight and lifts), yet you're complaining about a lack of muscle mass. I understand there is some variability between individuals, but that just does not compute. Maybe your body dysmorphia is getting the best of you.
To answer your questions, some body parts respond better to really high reps. Delts and legs, particularly. Don't be afraid of going over 20 reps a set. I did partials yesterday for side and rear delts (a drop set) and each set was well over 100 reps total. My delts have never responded better to anything than they do heavy partials for obscenely high reps.
Working one lift for strength (rep maxes though; nothing lower than 3-5 reps) and then doing volume work on everything else is pretty much exactly what I do. How much volume depends on you. You have to experiment. The amount of volume you're able to recover from is not a constant either, so you have to be willing and able to modify things as you go.