Welcome to the Hive, first and foremost
I think your exercise split is fine. I will note that 21 sets is a lot (that's rich, coming from me), so adjust accordingly, and shift an exercise to another day if you need to. I would also suggest incorporating some barbell work, if you have one - barbells are great "bang for your buck", if you will. Also, if you're looking to lift more over time, then you will need to adjust your weights and reps over time (as a powerlifter, this is what I do). But for now, what you're doing is fine.
But here's the truth of the matter.
Gaining muscle is mostly about what you eat. If you're challenging your muscles sufficiently in the gym, then the rest is up to what you're eating.
I forget what the guidelines are for protein intake, but do a search and figure out what changes you need to make to your nutrition. I recommend protein powder or a weight gainer - not as a shortcut, but to literally supplement what you're already eating. So take stock of what you're eating, and if you consider yourself a "hard gainer", then you need to out-eat your metabolism.
Hope this helps!