Since it’s your first cycle, I would probably finish your first 12 weeks with just test and then do a pct. Deca isn’t going to miraculously heal your back - all it does is make your body retain fluid around the joints and that’s usually why the pain can lessen taking it. It doesn’t fix or actually solve anything.
Go find out what your back problem is properly and get it treated - may be something that can just be fixed by a Physiotherapist, chiro, or Massage Therapist.
You say you’ve made great gains so far (i don’t know why you would gain any fat from a first test cycle to be honest if your diet is on point…if anything you should lose fat and quite a bit of it)…the 10lbs gain is likely water, some muscle, and fat.
Stick with the test for this cycle and make sure the other factors (diet, training, and injury) are on point. Get some blood labs to see how this first cycle impacted you and then you can consider other compounds for future.