You can't just "declare" domicile. Your state of domicile is determined by state rules and regulations. See page 2 of the reference @SweetieJean gave you to see how NJ defines domicile. You can have only one domicile at a time.
If you changed your domicile during 2021, then for 2021 you are a part-year resident of each state.
If your domicile (by NJ's definition) remained in NJ all of 2021, then for 2021 you are a resident of NJ and a non-resident of FL.
If your domicile was in FL all year, then for 2021 you are a resident of FL and a non-resident of NJ.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
