Well, I am not against on how you make your seed phrase safe but if you decided to import it on a wallet provider then there's still a chance no matter how small it is that your wallet will get stolen by a hacker. The only thing that keeps it safe is when you keep a back up and instead of storing the normal seed phrase is you make it encrypted it in your own way (kinda as you have said) but you would still decrypt it and import it when you want to use the wallet and you won't know that your device might have malware that can steal your seed phrase. I can't crack it that's my answer.