Polyethylene terephthalate (PET) is a widely adopted thermoplastic whose environmental accumulation poses significant sustainability challenges. While mechanical recycling is common, impurities often limit its efficacy, leading to increased interest in chemical recycling, or depolymerisation, to recover constituent monomers like terephthalic acid (TPA). However, the chemical inertness of PET typically requires harsh reaction conditions that undermine the environmental benefits of recycling. This study investigates the depolymerisation of PET under mild conditions using Lewis–Brønsted acidic deep eutectic solvents (LBDES). These systems, comprising iron (III) chloride hexahydrate and organic acids such as methanesulfonic acid (MSA) and para-toluenesulfonic acid (pTSA), serve as effective catalytic mixtures for ester hydrolysis. Research focused on comparing four depolymerisation approaches: standard thermal, extended thermal, ultraviolet (UV)-assisted (365 nm), and microwave (MW)-assisted methods. UV/Vis spectroscopy characterisation revealed that the LBDES systems exhibit maximal absorption in the ultraviolet region (240–270 nm), suggesting potential for light-facilitated reactions. Experimental results showed that the UV-assisted method significantly enhanced TPA yields, particularly in the MSA system, which achieved up to 93% yield with high purity. Conversely, while MW reactions were found to be the most energy-efficient despite lower yields, the UV method produced the lowest E Factor, indicating superior mass-to-waste performance. To ensure rigorous evaluation, this work introduced modified green metrics (ϵ′ and ζ′) to accurately account for the energy requirements of light-assisted and microwave sources. These results highlight the potential of LBDES as a sustainable pathway for PET recycling and provide a robust framework for evaluating future energy-assisted chemical processes.