Version 2 Update: This updated manuscript provides a more rigorous theoretical derivation of the domain-wall network dynamics. It includes a full second-order perturbation analysis using the ADM formalism to demonstrate the exponential damping of isocurvature modes, ensuring strict consistency with CMB constraints. Abstract: We present a cosmological model where an initially symmetric two-field scalar sector undergoes spontaneous symmetry breaking, leading to the formation of a transient domain-wall network. We derive the background dynamics showing that $\rho_{DW} \propto H$ during a metastable scaling regime, which provides a late-time enhancement of the expansion rate to alleviate the Hubble tension. The same mechanism produces a stochastic gravitational-wave background in the nanohertz band, correlating a percent-level shift in $H_0$ with a GW amplitude $\Omega_{GW} \sim 10^{-9}-10^{-8}$.