Hi Roger,
What is most likely happening when you feel your back more when doing more than 3 reps, is your form is simply breaking down on the later reps. I would highly recommend you start videoing your sets and watching them in between so you can see where the break down happens. If you can do 3 without pain, you should be able to find a weight that would allow you to do 4s or 5s so long as you figure out where your breakdown is, however I don’t see an issue with simply building volume with more sets at 3 reps especially if you’re just getting back into it and have more of a technique focus. My gut feeling says you’re probably losing your wedge at some point and that is why you feel your low back.
On the support work, separating the main and accessory work certainly is an option, but I wouldn’t say that this is absolutely necessary simply due to the fact you’ve had an injury. Find movements that don’t aggravate it and then adjust load so that your form doesn’t break down for the session. Remember core endurance, not just strength is a big component of good spine hygiene so having some length in your sessions will have a benefit so long as you remain mindful of your technique.
