Hi Peter2 and thanks for the post. I might start by saying because you don't feel the right hand side it might well be that is because it is functioning normally and not in a dysfunctional way. The large bulk of muscle that you might 'see' in that area is often the lats and not so much the erector spinae as they run vertically like cables. Of course you see over the top the various sections of the trapezius and if you're bulked up then some rhomboids as well.
I have found in weight lifters that the way to reducing that fatigue sensation in the trunk is to focus more on the abdominals and glutes/hamstrings in activities like bent over rows, deadlifts etc. They need to fire up as you lean forward and if they are not then you end up with a long bulky level hanging on the small musculature of the lumbar spine. In addition to that post around 90 degrees of forward flexion the erector spinae muscles switch off (myoelectric silence on EMG), perhaps as a safety mechanism. That means the support of the trunk relies on the abdominal complex (especially the obliques & transversus) and then the gluteals and hamstrings to work eccentrically and then concentrically as you move past 90 degrees and the come back up from the floor.
Without assessing you in person it is hard to be super specific suffice to say nearly everyone could do with dropping the weight and focusing on the pelvic movement on the hip joints in these types of movements.
As for the warmth sensation that might well be a small restriction/compression at the base of the next as the Cervicothoracic junction. You'd want some deep tissue work on your back in combination with some regular releasing on a ball and or foam roller to help maintain that movement.
I hope this provides some perspective on your situation and a way to self explore a little more.
regards
PBJ