This study investigates the complex dynamics of heat transfer and phase change within a flat-shaped thermosyphon, examining various parameters and working fluids. The research explores the effects of different working fluids, filling ratios, and heat inputs on the system's thermal performance. This study employs an axisymmetric model, to enhance the accuracy of the solution compared to a 2D model while maintaining a low computational cost. To validate our numerical model, we conducted comprehensive comparisons with experimental data, including the axisymmetric model, and capturing the nanofluid effects. Remarkably, our simulations consistently align with the experimental findings, demonstrating a high degree of accuracy and reliability in our model. The findings show that self-rewetting nanofluids exhibit superior heat transfer capabilities and a reduced risk of dry-out compared to traditional working fluids. Notably, self-rewetting fluids and self-rewetting nanofluids exhibit a unique behavior, reversed surface tension trends with rising temperatures, which effectively prevents dry-out by attracting liquid to hot regions. Additionally, the enhanced thermal conductivity of nanofluids during boiling enhances their performance furthermore. Also, the research studies the impact of filling ratios on temperature profiles and heat transfer rates, emphasizing the importance of higher filling ratios in lowering bottom plate temperatures. These insights offer valuable guidance for optimizing thermosyphons in applications like electronic cooling.