Using 5 x 3 is for strength. The volume isn't that much.
The purpose of 5/3/1 is to hit 5/3/1 not paralyze yourself over which BBB assistance you should use
If your training partners think 5 x5 is too much, then they shouldn't do it. Do 5 x 10 x 60% or 5 x 5 x 70% of the training max which should be 90% of your true 1 rep max.
Even if you do 5 x 5 at 70%, you will still build strength.
Stop overanalyzing, it's not going to make a difference if you do 70 or 80% just hit the 5/3/1.
Jim lays out a number of different BBB programs, try one, stick with it for a cycle or 2. If it works, great, it not, use another.