Janus engineering provides a promising route for converting two-dimensional (2D) antiferromagnets into altermagnets, yet the implications of the resulting symmetry breaking and built-in electric field remain poorly understood. Using first-principles calculations, we identify Janus $\mathrm{M}{\mathrm{n}}_2{\mathrm{P}}_2{X}_3{Y}_3$ ($X\ne Y=\mathrm{S}$, Se, Te) monolayers as a multifunctional 2D altermagnetic platform. We find that the built-in electric field induces a valley-contrasting Rashba effect, which we elucidate through a $k·p$ model incorporating the second-order spin-orbit coupling term. Crucially, spin-group symmetry enforces spatial decoupling of charge and pure spin shift currents, allowing dynamic switching of their directions via light polarization. Furthermore, strain engineering drives a semiconductor-to-metal transition in telluride members, accompanied by subsequent magnetic easy axis reorientation. Our study establishes $\mathrm{M}{\mathrm{n}}_2{\mathrm{P}}_2{X}_3{Y}_3$ monolayers as theoretical platforms for exploring tunable optospintronic and valleytronic physics, and provides symmetry-based design strategies for future practical device applications.

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• Spintronics
• Valleytronics
• First-principles calculations