I'm not a professional, but I was taught by one and have helped do the work on every home we've repaired/redid for 52 years. @DeVonSwi is right. Plywood instead of particleboard is definitely the way to go on flooring. I've seen particleboard fall apart far, far more often than plywood. Shimming is also the way pros used to level things. Cardboard shims disintegrate faster than wood.