Well exactly --but I think it is far different from the hardware wallets, correct me if I am wrong but hardware wallets have a security feature that can protect themselves against malware infection or an OS that is already infected with malware.
You certainly want that to be the case, but the reality is that almost no one can independently verify that is the case, and there could well be other attacks we simply don't know about yet which are still able to bypass any protections in places.
Essentially, what I wanna say is that I believe that running your own paper-wallet generator offline is the best method in terms of privacy and security.
What do you think ?
I think that 99.99% of people who try to design their own paper wallet generator will end up with something insecure. Your method combines a brain wallet, which are very insecure, with SecureRandom, which has also suffered from critical vulnerabilities resulting in people having their coins stolen -
https://www.theregister.com/2013/08/12/android_bug_batters_bitcoin_wallets/.
A far safer option to generate raw private keys would be to use Bitcoin Core. If you don't want to use a piece of software, then flip a coin 256 times.
Actually a private key is nothing more than a 256bit random sequence (e.g. 010110...1001)
The words that you refer to, derive from this 256bit number, if you add 8 bits more (checksum).
Therefore, it is quite simple to get the words, like you mention above.
You are confusing separate concepts here. A seed phrase does not encode an individual private key. A seed phrase is used to generate a near unlimited number of private keys in a deterministic manner, meaning backing up the seed phrase backs up all the private keys that it generates.