How to make totally secure Bitcoin transactions?
The primary concern with Bitcoin security is where your private keys are stored. 

In your scenario the private keys are stored on a Linux laptop which is not unreasonable if you follow good internet hygiene and security practices.

The next best approach is to purchase a hardware wallet from the likes of Trezor or Ledger. These wallets have High Security Module (HSM) chips inside them that are highly resistant to being compromised. By highly resistant it means the private keys are stored on a tamper-proof chip and NEVER leave it. The HSM chip in the Ledger Blue is ST31. Google for ways to compromise the ST31 versus ways to compromise Linux and you'll get the idea. 

With a hardware wallet it works in conjunction with a client on your PC but the crucial factor is the transaction (tx) is signed in the HSM chip and the only thing an attacker could do is try and replace the Bitcoin address in your tx. Most hardware wallets safeguard against this by displaying the recipient address on the screen when requesting verification of the signature operation.

And finally you could write your private key down on a piece of paper and hand-sign all your Bitcoin tx's. No electronic compromises to worry about.