An Improved Sinh Cosh Optimizer (ISCHO) is proposed to address Load Frequency Control (LFC) challenges. The enhancement over the original Sinh Cosh Optimizer (SCHO) addresses a limitation of its bounded search strategy: while improving search efficiency, it increases susceptibility to local optima. ISCHO mitigates this by expanding the population in subsequent iterations employing this strategy, thereby enhancing global search capability and better utilizing the solution space. The proposed ISCHO technique is rigorously evaluated using 13 benchmark functions (both unimodal and multimodal), demonstrating superior performance compared to Particle Swarm Optimization (PSO), the Sine Cosine Algorithm (SCA), and the Grey Wolf Optimizer (GWO). Concurrently, a novel controller structure, the Two-Degree-of-Freedom Double Integral Feedback Controller (2DOF-PIDF-II), is introduced as an advancement over the standard 2DOF-PID controller. The efficacy of ISCHO-optimized 2DOF-PIDF-II controllers is investigated for LFC in a realistic two-area multi-source interconnected power system. Each area comprises conventional generation units (thermal and hydro) and renewable sources (wind and solar). Comprehensive simulations incorporate critical system nonlinearities (Generation Rate Constraints GRC and Governor Deadband GDB), inherent system uncertainties, and load fluctuations. The performance of the proposed ISCHO-tuned 2DOF-PIDF-II controller is benchmarked against prevalent control techniques (PID, PIDF-II, 2DOF-PID) and established optimization algorithms. Simulation results conclusively demonstrate that the ISCHO-based 2DOF-PIDF-II controller significantly enhances system frequency stability under diverse and challenging operating conditions, including varying load disturbances, system uncertainties, and physical constraints.