I'm using, Trust wallet, and Bitkeep both are good decentralized wallets and also have a working decentralized exchange but regarding that gas cost, I don't think there's an app that can help you. only one Platform I know that can reduce the Eth/Erc20 withdrawal fees and it was not a DEX but a centralized exchange called FTX, usually, they shoulder half of the transaction cost. and also giving a one-time free eth withdrawal per day if you stake their token FTT. Anyway, it's just an additional suggestion you can ignore it if you don't want to use a CEX.