This is the first I've heard of thrusting with a 3.5lb staff. Where did you get the staff and what is it made of?
Doesnt matter where you get the staff, if its about as long as you are tall and is made of solid wood (no waxwood) then its sufficient for spear thrusting.
Working out with simple polearm type weapons help to "correct" the body if you know how to work it.
Spear thrusting is one way to approach it
So, why thrust a spear to train the body?
Cuz you can't use "strength" to thrust it.
The mechanics of "spearing" someone are fundamentally different from "sport" movement that you might find in modern day boxing.
It requires a complete understanding of how to shift weight (Shioda Gozo's favorite "taijuuidou"), how to open and close the pelvic girdle(kua area for CMA fetishes), and how the tanden/koshi passes force to the tip of the spear etc etc