Are there DeFi platforms that allow instant access to staked coins?

You'll have a hard time finding a DeFi platform that allows such a feature. Even if you do find such a platform, they'll probably be an unreliable one with potential security issues. The truth is that the DeFi space has such competitive interest rates with staking due to the fact that you cannot access the tokens in the meantime. Make sure that you're only staking for the long-term. If you're goals are short-term gains, then staking probably isn't the right choice for you.