For very selective schools you should have 700+ scores. They may consider you if you are a STEM student with 750+ math and high 600 for reading or vice versa if you are a humanities student.
Are you a senior or junior? Most people do not take SATs until spring of junior year. If you are a senior then you have no choice but to take the test now, otherwise I would wait if you do not feel comfortable. I don’t think you are going to do better than your practice tests because you will be more nervous during the exam.