Chairs for your sewing area is very important to avoid back pain, along with the short breaks where you get up and move around. Ergonomics and stretching are big in my workplace, and in many other work places, so it makes sense in our homes at our sewing machines too. I try to be aware of my body while sewing, shoulders and position to the needle like you mentioned. When I have an aware moment of my body, I often find my shoulders scrunched right up tight no matter how much I try to keep them relaxed. The lumbar supports like you mentioned you bought are used alot at my work place, and people do report relief of lower back strain, which makes sense at a sewing machine too, hope it helps you too.
I have read threads on this board about chairs and other options. I currently have a swivel office type chair that I can adjust the height on. I have also recently added bed risers to my table, and can actually stand up to sew now also, which has become my preferred. I had my haircut Thursday, and my beautician just bought a saddle seat, which she let me try out. I've read about people on here having them and the majority of them report they really like them for various reasons. I'm considering one for use at my mid-arm machine. I was just happy I was able to get on and off the seat without falling, I'm not the most graceful person.
When your comfortable and have less body aches when sewing, more projects get done, and obviously the quilting world has realized this, because when you go to one of the larger quilt shows, there are ergonomically correct rotary cutters, tables that adjust height, ergonomic chairs, heated and vibrating lumbar supports,cushions to sit on, and the list can go on. Quilting is fun, rewarding, and enjoyable, ect..... so be sure you take care of your body while doing it so we can do it for many years to come.