Use a SHA256 as a key for AES256 Is it safe to use the SHA256 of a hard-to-bruteforce string as the key to encrypt data with AES256?
Yes, it is safe. The answer of @kelalaka (the first part, before update) it not correct.

The OP is saying that the password is a strong string which is hard to bruteforce. That's why the answer of @kelalaka the attacker, firstly, will try to brute-force is not relevant to the question. Everything @kelalaka is saying about password stretching is correct, but it is not relevant to this particular question.

I am curious why would you want to use SHA256 instead of password, if your password is complex and is hard to bruteforce. But this is another story.

I would suppose that some readers who didn't read the question attentively will vote  my answer down. Some people don't think much and just apply patterns that they have in their minds, like Password? --> hashing is bad --> use stretching instead But guys, read the question once again.