This document presents v21.0 of Relaxation-Driven Cyclic Cosmology (RDCC), a minimal and nonsingular cosmological framework where the Universe undergoes an infinite sequence of smooth contraction and expansion phases. Formulated entirely within standard General Relativity and a single complex scalar field, RDCC incorporates the leading Planck-suppressed interaction between two CPT-conjugate sectors sharing the same spacetime metric. This two-sector structure replaces the assumption of a globally directed time with a CPT-symmetric description, making thermodynamic arrows, entropy production, and matter-antimatter asymmetries emergent, sector-dependent phenomena. The global system remains entropy-conserving, resolving the Tolman entropy problem without inflation, modified gravity, or additional fields. The scalar potential features a radiatively tilted double-well structure augmented by an ekpyrotic term dominant during contraction. Combined with bounded effective Friedmann corrections from the EFT expansion, these elements produce a smooth, ghost-free bounce with finite curvature invariants. The ekpyrotic phase generates a nearly scale-invariant spectrum of curvature perturbations, while the Planck-suppressed cross-sector interaction damps isocurvature modes as a^{-α}, driving the Universe toward near-adiabaticity. A key outcome of the relaxation mechanism is a temperature asymmetry between sectors, naturally yielding the observed dark-matter-to-baryon ratio through exponential sensitivity to the relaxation parameter α. This parameter also induces small, correlated deviations from ΛCDM in growth rates and the matter power spectrum, plus a modest ∆N_eff contribution. RDCC provides a unified explanation for nonsingular evolution, dark matter, baryon asymmetry, the thermodynamic arrow, and primordial near-adiabaticity. Its predictions, derived from a single infrared parameter, are testable with upcoming large-scale structure and CMB experiments, offering a structurally simple yet observationally rich alternative to standard cosmology. Changes from v20.0: Enhanced emphasis on EFT consistency with dimension-8 operators and positivity bounds; explicit resolution of the near-adiabaticity puzzle via isocurvature damping; refined perturbation theory and phenomenological predictions. This version builds on: Relaxation-Driven Cyclic Cosmology (RDCC) v1.0: A Minimal Eternal Framework and Its Dynamical Realization: https://zenodo.org/records/18281686 Relaxation-Driven Cyclic Cosmology (RDCC) v2.0: A Minimal Eternal Framework with Dynamical Realization and Extensions: https://zenodo.org/records/18304744 Relaxation-Driven Cyclic Cosmology (RDCC) v5.0: Bounce-Matching Formalism, Mirror-Sector Dynamics, and Refined Gravitational Waves: https://zenodo.org/records/18307836 Relaxation-Driven Cyclic Cosmology (RDCC) v6.0: Black Hole Bounces as Local CPT-Fixpoints, Full Perturbation Transfer Matrix and Mirror-Sector Microphysics: https://zenodo.org/records/18312656 Relaxation-Driven Cyclic Cosmology (RDCC) v7.0: Quantum-Corrected Black Hole Bounces, Full Perturbation Transfer Matrix with Phase-Shift, and Mirror Microphysics https://zenodo.org/records/18327084 Relaxation-Driven Cyclic Cosmology (RDCC) v8.0: Parameter Space Exploration, Data Confrontation, and Predictive Structure Formation https://zenodo.org/records/18334398 Relaxation-Driven Cyclic Cosmology (RDCC) v9.0: Predictive Cosmology, Nonlinear Structure Formation, and Astrophysical Signatures https://zenodo.org/records/18336237 Relaxation-Driven Cyclic Cosmology (RDCC) v10.0: Unified Cosmology, Full Simulations, and Observational Pipelines https://zenodo.org/records/18336865 Relaxation-Driven Cyclic Cosmology (RDCC) v11.0: Entropy Dynamics, Mirror Thermodynamics, EFT Consistency, and Naturalness https://zenodo.org/records/18341147 Relaxation-Driven Cyclic Cosmology (RDCC) v12.0: Precision Predictions, Multi-Probe Confrontation, and Quantum-Geometric Completion https://zenodo.org/records/18346555 Relaxation-Driven Cyclic Cosmology (RDCC) v13.0: Quantum Completion, Global Consistency, and Predictive Cosmology https://zenodo.org/records/18348450 Relaxation-Driven Cyclic Cosmology (RDCC) v14.0: Unified Quantum Gravity, Full-Cycle Simulation, and Multi-Messenger Precision Cosmology https://zenodo.org/records/18351916 Relaxation-Driven Cyclic Cosmology (RDCC) v15.0: Mirror-Sector Cosmology, Hidden-Universe Gravitational Waves, and Two-Sector Evolution https://zenodo.org/records/18358095 Relaxation-Driven Cyclic Cosmology (RDCC) v16.0: Precision Two-Sector Dynamics, Mirror Thermodynamics Refinements, and Enhanced GW Forecasts https://zenodo.org/records/18388123 Relaxation-Driven Cyclic Cosmology (RDCC) v17.0 - Minimal Effective Two-Sector Version https://zenodo.org/records/18761806 While the core of RDCC remains a structural, timeless framework with fixpoint character, this document explores a dynamical realization of the bounce mechanism to enable concrete, falsifiable predictions. Related documents and the full RDCC ecosystem are available via Zenodo:https://zenodo.org/records/18204087 Michael Lehmannmi.lehmann@gmx.de