As renewable energy penetration rapidly increases, Independent System Operators (ISOs) face unprecedented challenges in balancing cost, emissions, and reliability. This study introduces an Integrated Grid Optimization (IGO) framework, combining siting, transmission expansion, market dispatch, and inverter stability solutions. Using publicly available datasets and the open-source GridPath platform, we simulate coordinated scenarios (30%, 50%, and 80% VRE) from 2025 to 2030 across MISO, CAISO, and ERCOT. Results demonstrate that the IGO framework can reduce CO₂ emissions by up to 80%, maintain high reliability, and minimize total system cost through optimal placement of HVDC overlays, battery storage, and grid-forming inverters. This provides a replicable blueprint for cost-effective, stable renewable integration across large grid footprints.