The Laser Interferometer Space Antenna (LISA) will open the millihertz band of the gravitational-wave spectrum, enabling observations of massive black hole mergers and extreme mass-ratio inspirals. The mission’‘s success depends critically on the signal detection and processing chain (SDPC), which encompasses the instruments responsible for optical detection, filtering, and interferometric readout. Within the SDPC, the phasemeter must measure optical pathlength variations with picometer precision while simultaneously executing a suite of auxiliary functions, including clock distribution and absolute ranging. Verifying this multi-functional performance to the required precision poses a formidable challenge from a ground-based perspective. We present Hexagon-2, a next-generation optical testbed achieving picometer-level stability for comprehensive validation of the LISA SDPC. Building on its predecessor’‘s feasibility demonstrations, Hexagon-2 features an optical bench redesigned around suppressing residual tilt-to-length coupling noise and polarization leakage, a novel kinematic mount for enhanced thermo-mechanical stability, and an architecture that enables the direct integration of flight-like hardware. Hexagon-2 forms the optical core of a scalable, end-to-end flight hardware validation facility, providing a critical de-risking step for one of the mission’’s most demanding subsystems.