This work presents the design, synthesis, and application of advanced doped and composite graphitic carbon nitride (g-C₃N₄)-based photocatalysts for the efficient degradation of diverse organic pollutants under visible light. Sulfur, yttrium-, tellurium-, and strontium-doped g-C₃N₄, as well as 2D Z-scheme S-g-C₃N₄/WO₃ composites, were synthesized using facile thermal polymerization, two-step pyrolysis, one-pot calcination, and wet-impregnation methods. These materials exhibited significantly enhanced photocatalytic performance compared to pristine g-C₃N₄, achieving up to 99% degradation of industrial dyes such as methylene blue, acid orange 7, and congo red, as well as pharmaceutical contaminants including acetaminophen and 5-fluorouracil. The reaction kinetics followed pseudo-first-order models, with substantially higher rate constants and shorter half-lives for the doped and composite catalysts. Mechanistic studies, supported by radical scavenger tests and LC-MS analysis, revealed superoxide radicals and photogenerated holes as the primary active species, with minor contributions from hydroxyl radicals and electrons. Structural, optical, and electrochemical characterizations confirmed improved charge separation, band gap tuning, and stability of the synthesized photocatalysts. Biocompatibility tests using seeds demonstrated negligible toxicity and potential applicability in environmentally friendly water treatment. These findings underscore the versatility and efficiency of doped and composite g-C₃N₄ photocatalysts for sustainable remediation of various organic pollutants.