The development of efficient flight software (FSW) for CubeSats faces significant challenges due to lack of mainstream guidelines and frameworks. Addressing the challenges of developing such FSW within the ever-evolving yet stringent hardware constraints forms the core of this study. This paper presents a comprehensive analysis of FSW specifications and development challenges and proposes a novel approach to software design that maximizes functionality while adhering to hardware limitations. It begins by scrutinizing current trends in CubeSat FSW, emphasizing the significance of modularity and reusability for successful, adaptable, and repeatable space missions. Drawing from a diverse array of literature sources, various aspects of CubeSat FSW are explored, encompassing design methodologies, subsystems, mission applications, reliability, fault tolerance, testing, validation, and communication. Subsequently, a case study is introduced featuring an application-based FSW solution tailored for a 12U CubeSat with a 5G Internet-of-Things payload and green propulsion. This case study incorporates insights from the literature review, featuring a service-oriented architecture. The FSW solution includes a user-friendly command line interface for efficient onboard and ground communication, a script engine for timely in-orbit execution and payload control, and a bootloader for in-mission updates, enhancing adaptability and future readiness. The analysis and case study reaffirm the pivotal role of robust and efficient FSW in mission success, while underscoring the need for freely available, modular, and reusable solutions to foster innovation in the field, ultimately reducing reliance on commercial products or continual redevelopment.