Well a serious enough lifter to do 6x full body is probably taking some mix of fish oil and glucosamine.
I can't imagine any circumstance, beyond negatives or rep-max testing that warrants going to failure.
Don't change the volume, merely redistribute it; instead of 2 sets on the back each session, you're doing 1 set, twice as often (or halving the # of exercises).
That last split is akin to something I used to do, but alternating squats and deads. At actual 5RM I would do it like this:
M: Squats, Incline, Chins
Tu: Deads, Dips, Rows
We: Incline, Chins
Th: Dips, Rows
F: Squats, Incline, Chins
Sa: Deads, Dips, Rows
Su: Sweet **** all
2 sets, not a rep more.
I did calf raises every day and if I felt a particular muscle group had worse endurance than it's opposing group then I would add an isolation exercise to assist in that regard.
After the 15s, lifting fatigue will probably not trouble you again, ever. The body adapts and your CNS will just fall into line.