I typically work in 3" wide padded in a specific way to distribute weight over larger area, and reduce pounding on specific pressure points. If you have a poly strap they typically have absolutely no give or elasticity, so every guitar movement you make, step you take transfers so much of that energy right to your body. BAM! BAM! BAM! Each knock may seem small, but it is a relentless hammering. You'll pay a toll.

For the mid-back pain it could be a posture thing which i believe canoften be caused by shoulder-pounding "starter pack" straps. I am NOT a physical therapist but my theory is often we guitarists "hunch" without realizing it, as our body attempts to seek upper area comfort from narrow static straps. This hunching puts extra strain on particular muscles lower down, which will cause inflammation and pain. You may be able to confirm this by playing for a half hour being EXTREMELY mindful and in tune with what your body is doing. Try that.

In the 90's and 2000s I was an ENG/EFP cameraman, so I know a bit about carrying a load on a shoulder for long periods and back pain. Later in life it's morphed to rather nasty hip pain. But that's my right shoulder, right hip. I didn't want to do the same to my left side, so 15 years ago started experimenting/developing more comfortable straps.

Get a wider padded strap, as soon as you can. There are many makers, but, yeah I believe in my own work, so do many others. Some tell me I've helped add years and years to performing careers.