hey Roger,
I am 100% behind what my teammate Andrew is saying. I would reinforce the idea from personal experience on both myself (coming back from spine injury) as well as my applying this to my lifters ,about the importance of avoiding pain triggers in both the main movements and accessory work.
Other options on your sets and reps is to work singles for several sets. For example, 20 sets of 1 gives several reps to set up perfect form and a tight lifters wedge while simultaneously giving you 20 opportunities to train your body to perform on the neurological level. As Andrew said, a form break down due to fatigue is a very plausible reason for pain above 3 reps and both trunk endurance/strength could be a culprit. I personally have found that deadlift and the appropriate accessory work in the same workout has worked wonders. Take note that in spine injury what is optimal for one person may cause pain triggers in others, so be very picky about what you do. Make sure to really identify what works for you and start creating a list of optimal movements as well sets and reps that have the most positive cost to benefit ratio on your spine. Stay pain free and I hope this helps.
P.S.:
I post several of the movements that have helped me come back into Strongman Pain free on both my training log and my Instagram (@thebattleaxegym). Stay safe brother.
cheers,
MDLP