To some extent, DEX is safe, but I would not trust them with the private key from my main wallet, which stores valuable tokens and ETH. It is better to create a another wallet for DEX, which is used to buy and sell tokens, but not for hold tokens, because you need to take into account that in case of a hacking of the exchange your private key will be stolen.
That is not bad step to take as security measure for safeguarding our investment, there are lots of things that scammers can do and even though we know that dexes are the safest when it comes to hacking of account because all coins are not stored in one place and wallet only get involves when you decide to link it, but since hackers are always online too 24 hours in a day, before you can be fast enough to disconnect your wallet, they might have gained entrance to such wallet.
The best measure is just to do what you said, have different wallet for some trades that we do in dex, just like I usually do for my mails too. I don’t use same mail where I have my keys to log in on any of these project sites, I simply created something like a junk mail to be on a safe side.