Thanks for reading the sticky and answering the Qs, even if you made a helluva long as post. Still prefer this to the absolutely context-less questions we usually get.
Let's talk about a few things.
First, your back. You say:
This is not good sir, not good at all. Novices sometimes do describe things in ways that they'll later laugh at when they gain more experience, so it might just be your lack of context, but it's still enough to worry me. Your back should not feel like this, at all. Fatigue, sure. Arthritic at 37, when your squat and DL are only in the low 300s at 255 bodyweight? No.Lower back feels how I imagine arthritis to feel all of the time. Hard to put on my socks while standing, hurts too bad to progress furthur in squat and deadlift
I'm going to make two guesses:
1. You need a form check on your squat and DL, pronto, and we will likely find some back related stuff that needs to be cleaned up...
and/or
2. You are still deadlifting and trying to add 5 lbs too frequently, and probably in the squat, too (still trying to grind out 3x/week increased of 5 lbs, even on a huge caloric deficit, instead of accepting reality and switching to advanced novice).
I am willing to bet a Coke Zero, to quote my friend Comrade Campitelli, that at least one of those two things is in play here. And wouldn't be at all surprised if both of them are.
Second, let's talk about your nutrition. It's true that a calorie restriction / eating at a big deficit - and 1500-2000 cals a day is going to be a pretty big deficit for a 255 lb man - will shorten your LP. If you need to lose the weight, and you probably do, then a caloric deficit is fine. But I'm a little suspect here. How could you have been eating 1500-2000 cals a day for 2 months, doing SS, and only have gone from 265 to 255. Something doesn't add up.
My inclination would be to try something a little less deficit-y and see how you do (like 2000 on off days and 2500 on training days), but first you need to clear up some other things. Have you had issues with compliance? I just don't think you'd only have lost 10 lbs in nearly 9 weeks, your first 9 weeks especially, on that kind of deficit.