To the OP, something I learned the hard way, and which you are learning now, is that recoverable volumes at a certain percentage of 1RM do not track linearly. In other words, the stresses from a 75% 5x5 at a 1RM of 300 are much, much less than the stresses from a 75% 5x5 at a 1RM of 700, even though on paper they are the same relative volume.
There's a reason you don't see 1,000lbs squatters doing 5x5bwith 750lbs.
I have tried (more than once, embarrassingly) to get from a 650lbs squat to a 700lbs squat by structuring my training the way I did when I went from a 450lbs squat to a 500lbs squat, and failed miserably. I eventually learned what Mark or any other experienced coach will tell you, that the stronger you get, the less relative training stress you'll be able to recover from and adapt to.