If I use a decentralized exchange and a wallet, such as ledger nano, this would be the safest possible way to avoid hackers ?
Hackers, yes. Muggers/kidnappers, maybe not. (For absolute security in such circumstances you'd be looking at multi-sig and keeping the keys in ultra-secure locations. But that's probably overkill for 99% of us.)
Keep in mind some wallets have features like the ability to host multiple seeds, such that you can trick a kidnapper with a secondary address that only has a small portion of your funds.