Photo-electrochemical, bandgap selective, lateral etching is used to create 200 nm-thick, ultra-smooth GaN membranes, containing 10 pairs of GaN/AlGaN quantum wells. The use of electrolyte concentrations as low as 0.0004 M, along with appropriate excitation power and bias conditions, are shown to enhance the quality of freestanding membranes immensely, with an AFM roughness of 0.65 nm; the best ever reported value for GaN membranes fabricated using a similar technique. Transmission and photoluminescence experiments on these membranes were made possible at cryogenic temperatures by membrane transferring onto a double-side polished sapphire substrate, revealing pronounced excitonic features; the analysis of which strongly suggest that the absorption coefficients of GaN are at least 30 % higher than the values previously reported in the literature.