For hypertrophy, it’s important, but it’s not the most important factor. If all you had was a fixed-weight, you could still build plenty of size by manipulating volume (sets and reps) and adjusting time under tension (intensity techniques like rest-pause, negatives, partials, etc.).
Autoregulation, basically playing workouts by ear and adjusting set-to-set based on performance and feel. Discussed in a few different articles. Thib talked about it, there was a study supporting its benefits, and I discussed it.
Also, re-watch Pumping Iron. You won’t spot many notebooks in the gym.
As is always the case when people say this… dude, c’mon. You’re 6 feet tall and under 170 pounds. Your legs are not growing like weeds. You need to be training everything, not de-emphasizing anything.
As Paul Carter succinctly told you last month, “Right now you’re weak as hamster piss, so regardless of what you do routine wise you need to go from a 135x8 bench to more than that.”