IMC,
The exchange shop is an excellent idea to make people feel that their money is not wasted on Goddess Cubes, however you need to include items from recent cubes to keep it relevant.
There is too much garbage in the goddess cubes that ends up being swapped for exchange medals. You should be rewarding your players for investing in your game instead of punishing them with ridiculous gambles.
I would recommend including items like wings and armbands into the exchange shop. It shouldnt be a 1% chance with 30 TP invested for cosmetics, that is way too much to ask from your players and will not be sustainable.