I might have found something! Bailey Metal Products make lightweight steel framing wall studs and floor joists. I read through their load tables and found one of their 2x6 joists can match my span, spacing, load and deflection targets. I cross matched one of their 2x4 studs (albiet thicker gauage) that has the same or greater properties than the 2x6 and only weighs ~19lbs (comparable to spf). Would that work as a direct replacement?