The antecedent is the predicative of the main clause or the relative pronoun is the predicative in the attributive clause. (that) attributive clause

First of all, omit that as a predicative in the attributive clause.

When the relative pronoun is the predicative of be and the antecedent is concrete, the relative pronoun usually uses that (without who or which) or is omitted:

He doesn't seem to be who he was ten years ago. He doesn't look like he did ten years ago.

Modern airplanes are not the machines that were first invented. Modern airplanes are not what they were when they were first invented.

She is no longer the happy woman before she got married. She is not the beautiful woman before she got married.

She fully meets the standards of a teacher. She has all the qualifications that a teacher should have.

But if the antecedent is a general reference, the relative pronoun generally uses which, without omission:

Even though John is a millionaire, he is not, and he will not use his penny to benefit the people. Even if John is a millionaire, he won't use his penny to do something beneficial to the people. He is not a millionaire either.

He looks like a lawyer, and he is indeed a lawyer. He looks like a lawyer, and he is indeed a lawyer.

When it is used as a complement in a clause. For example:

I am not the fool you think I am. I'm not the fool you used to think.

He is the best teacher that the students in their school think of him. He is the best teacher in the school that the students think.

He is not the man he used to be. She fully meets the standards of a teacher.