Current RN here who is about to matriculate this fall. I did well on the MCAT (>90th percentile). Are you planning on working full-time while studying? I set aside 4 months to study, which changed to 5 months d/t COVID pushing my original date, and I worked full-time until the last couple weeks before the test. Working 3x12s gave me time to study, which is all I pretty much did when I wasn't working. I would also plan on having your test date happen at the right time so that you will be ready to fill out your primary application, which is a huge time sink as well.
I would only take the test after taking all pre-reqs, including biochemistry. I used the Berkeley Review for content review, as well as numerous internet resources. I finished the content review after about a month in a half and spent the rest of the time on practice questions through UWorld and Altius before transitioning to the official AAMC material closer to the test date. I kept a spreadsheet of questions I got wrong and reviewed the material to make sure I understood why I got it wrong in the first place. Take the practice tests in real conditions to get a picture of where you are at.
Message me if you have any other questions. My experience as an RN was a huge plus on my application and during interviews.