High-quality GaN epitaxy and transfer are essential for flexible optoelectronics. Yet, the trade-off between strong covalent-bonded interfaces for epitaxy and weak forces for exfoliation hinders the simultaneous achievement of high-quality epitaxy and intact transfer. To address this, we report a dual-functional metal-interlayer chemical lift-off (MI-CLO) strategy. To the best of our knowledge, this is the first demonstration of a metallic interlayer serving as both an epitaxial template and a dissolvable sacrificial layer, facilitating both single-crystalline GaN thin film growth and low-damage transfer to flexible substrates. Our results demonstrate that the copper (Cu) interlayer yields superior surface morphology and crystalline integrity of the epilayer than nickel, despite their structural similarities. By incorporating a Cu interlayer and composite buffer layers, we obtain a 33-µm GaN film on sapphire with the (0002) rocking curve full width half maximum as low as 0.10°. A 5-µm GaN thin film exhibits a low threading dislocation density (8 × 10⁸ cm⁻²) comparable to direct heteroepitaxial GaN films on sapphire. The MI-CLO technology demonstrates broad applicability across representative heteroepitaxial systems, facilitating strain-released transfer with high epilayer crystalline integrity onto flexible substrates. A flexible ultraviolet photodetector is demonstrated, maintaining stable responses under mechanical bending and verifying its potential for flexible optoelectronics.