Ah talmid,
You are simply incorrect. You are incorrect on two points:
1) If he knows of a mitzva understands it is binding but knows that his tayva is too great and he cannot keep the mitzvah. His giyur is valid. The kabola would only be invalid if he holds that he doesn’t have to keep a particular mitzva, that it is not binding on him.
2) If he verbally accepts the mitzvos than he is bound by his acceptance and the giyur is valid. He can’t come later claiming that in his mind he didn’t mean it. Dvarim Sh’bilev Eiynam Dvarim.