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It seems that there is some misunderstanding on your part of what MetaMask + Ledger do.
First, you should never reuse your MetaMask seed phrase in Ledger. Instead, you should generate a new seed on Ledger, connect it to MetaMask and let MetaMask derive the corresponding addresses from Ledger. So, you will end up with multiple addresses on MetaMask: some from the seed stored in MetaMask (hot wallet) and some from the Ledger (cold wallet).
Second, transfer all your funds from your hot wallet addresses to a cold one (managed by Ledger).
Third, in order to interract with some risky DeFi’s, I would indeed suggest creating a new address from Ledger (using MetaMask, not Ledger Live). This way, you can keep a safe amount of your crypto there, so even if a malicious smart contract will trick you into signing your tokens away, it won’t affect your main savings.
Fourth (last one), when you interact with a smart contract and is about to sign something using MetaMask interface, you can actually see what it is that you are signing. If a smart contract says in needs access to some scam tokens that it airdropped earlier into your account but instead wants unlimited access to all your ETH, MetaMask will absolutely show that. Most users do not bother to double check what they are signing/approving, and this is the second biggest problem (after willingly giving their seed phrase to a first stranger asking for it).