Question about gas fees

From what I understand, spending less gwei means a longer transaction time, and spending more gwei will shorten it. Personally I don't really care how long the transaction takes, I just want to pay as little in fees as possible. For some reason, coinbase wallet only lets me change the gwei sometimes. I have no idea what determines whether I can or cannot change the fee. When I try to send ETH it will tell me I don't have enough in my balance to cover the fee without even letting me get to the page where I can change it. I've been thinking about switching to metamask, but both wallets offer this feature so I'm not even sure it will solve my issue.