Water pollution by toxic organic compounds demands efficient photocatalytic materials that are easily integrated into water treatment systems. We report a novel Fe3+-doped layered double hydroxide (LDH)/polyetherimide (PEI) composite for the visible-light degradation of cationic dyes (methylene blue, safranin T). Four Fe-doped LDH samples were prepared via co-precipitation with variable synthesis procedures and pH levels. The optimal sample formed nanodiscs with the highest specific surface area (104 m2·g−1) and additional energy levels below 3.2 eV. Incorporating LDH into the PEI matrix improved mechanical stability and enabled complete polymer combustion upon thermal disposal, leaving only inorganic residue. Scavenger experiments revealed that steric constraints shift the dominant reactive species from holes for the degradation of planar methylene blue to superoxide radicals for bulky safranin T. The composite achieved 93% methylene blue removal and maintained performance over three cycles with high mineralization. Testing in real river water demonstrated that the composite retained up to 76% of its initial efficiency. The proposed strategy combines high photocatalytic performance, easy catalyst recovery, and reusability for sustainable water treatment.