To be honest, I am not sure if replacing BK with FW per se would make a big difference with regard to arm-friendliness (where you already experience arm pain). If you want to stay with full poly, I would first try to play the thinnest possible gauge (for example GW19g in the crosses and BK 18g in the mains, or a full set of GW 19g before moving to a multifilament string) and make sure not to play longer than 10-12 hours before replacing the string. Our belief is that many players play their poly strings too thick and consequently play it to the point where the string becomes "dead" material. Playing with ANY poly longer than 10-12 hours will increase arm harshness significantly!
Could not agree more with this. I track my time on strings pretty judiciously (notes app) and always restring poly around 8-10 hours depending on the string. Tension loss is less an issue than the poly going dead and putting a lot of strain on the arm.