The increasing demand for electricity and fluctuating renewable generation intensify congestion risks on the Dutch transmission grid, challenging the reliability ensured by TenneT, the national Transmission System Operator (TSO). Currently, redispatch and balancing are operated in separate markets, limiting efficient use of flexibility and increasing system costs. This study develops and evaluates Flexmeister, an integrated balancing and redispatch market design, where suppliers provide a uniform flexibility product that TenneT can allocate in real time. The simulations in this study were based on the actual Dutch high-voltage grid topology, physical power flow constraints, realistic generator locations, parametrised bidding behaviour and representative time-series data. To evaluate the performance of Flexmeister, four market designs were simulated: the Current (separated) design, Gross, Net, and All-in-one (one-step optimisation). Three market designs were structured around three sequential market rounds: DA, redispatch and imbalance, whereas All-in-one executed redispatch and imbalance at the same time-step. The benchmark results demonstrated that integrated designs consistently outperformed the current design across all key performance indicators: extra capacity, total costs, activated volumes and clearing prices up and down. Simulations consistently reported the performance in the following order: All-in-one > Net > Gross > Current. Unlike the current market, the integrated designs required no additional out-of-market capacity, thereby improving reliability without costly infrastructure investments. The findings provide the first quantitative evidence that market integration can strengthen grid reliability and lower societal costs. Moreover, the developed simulation model bridges economic market design and technical system modelling, offering policymakers and grid operators a practical tool to support the energy transition.