Large-scale agile (LSA) frameworks are increasingly adopted by financial institutions to balance rapid digital innovation with complex regulatory requirements. However, the systemic impact of these frameworks on developer cognitive capacity, psychological safety, and long-term socio-technical sustainability remains under-researched. This paper presents exploratory case study of a Swedish financial services organization navigating a Scaled Agile Framework (SAFe) transformation. We conducted fifteen semi-structured interviews across multiple organizational roles and analyzed the data using reflexive thematic analysis. Our analysis identify six recurring socio-technical tensions shaping developer experience and team functioning. Rather than representing isolated challenges, these tensions form an interacting system. We synthesize these findings to explain how coordination intensity, governance ambiguity, and delivery pressure recursively reinforce cognitive overload, defensive team behavior, and socio-technical debt accumulation. We argue that developers well-being is a leading indicator of socio-technical sustainability, not a peripheral human resources concern.