I'm looking for a progressive way to increase my work capacity without stalling my BIG lifts.

Background: I don't tolerate a lot of volume mixed with intensity in my workouts. Whenever I try to add too many sets or more exercises, then my main lifts start to stall FAST. Currently, I'm on 5/3/1, and when I tried to include "complexes" I stalled my squats and bench within two weeks. I think this is because I stuck to an abreviated routine for too long and didn't progress the way I should have in other areas.

Goals: After I finish reaching my goals for a strength base, I'd like to start working on hypertrophy, while still increasing my strength base. My strength goals for the short term are a bench of 300 lbs for a few reps, and a squat/deadlift with 400 lbs for a few reps. I am quickly approaching these goals.

Problem: Once I've reached the strength goals I want to start including some "sensible" hypertrophy specific work. However, I'm afraid that my work capacity won't be up to the task. So, I'm looking for ways to start increasing my work capacity in a gradual manner so I won't interupt my progress on the strength gains. I would appreciate hearing your ideas and experiences with this.