Getting speaker placement right takes some time and patience. I do recommend trying to get speaker placement right without any room treatment first, for when you get the speakers in the best location you will notice less room interaction. Also you will achieve the best bass responce in the right location. The 1/3 rule does give you a good starting point and then move the speakers around concentrate on first getting the bass responce right first.
As for treatment and reflections keep in mind you only need to treat one parallel surface not both. Except on the side walls you want to keep the treatment of side wall reflections symetrical. You want to treat the back wall behind the listener, not the front wall, and then as little as possible on the side walls generally where the speakers first reflection back to the listener, ideally with some sort of defusion panels rather then absorbtion. Problem with absorbtion is its not linear in its absortion characteristics, meaning it will absorb at certain frequencys and reflexed at others causing comb filtering. I personally think you can acheive excellent result with typical materials we all may have around the house, light curtains, area rugs, etc. Try different materials starting with the lightest materials first, to get the desired results your looking for. Reflections are good they just need to be controlled and delayed. The difficult issues are bass nodes, which speaker placement will help in that aspect significantly.
Any other questions feel free to ask
40 years hi-end audio video specialist