Hey! I think firstly the relaxation on the backswing is very important, using the body turn and being relaxed will naturally give you more backswing. But I would also try to bring the arm back and open out the angle between the upper and lower arm like you mentioned, everyone does this slightly differently but I would say you can open it put almost fully and then snap through during the shot to finish about 90 degrees.
With your arm distance to the body it’s important to keep this consistent a good guide is the space the size of an orange between your arm and body, this leaves enough space so it’s not too much effort but also not cramped and becomes difficult to backswing freely. Hope this helps and let me know if you have any questions on that 🙂
